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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>pix2rule: End-to-end Neuro-symbolic Rule Learning</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Nuri Cingillioglu</string-name>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Alessandra Russo</string-name>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Imperial College London</string-name>
        </contrib>
        <contrib contrib-type="author">
          <string-name>United Kingdom</string-name>
        </contrib>
      </contrib-group>
      <abstract>
        <p>Humans have the ability to seamlessly combine low-level visual input with high-level symbolic reasoning often in the form of recognising objects, learning relations between them and applying rules. Neurosymbolic systems aim to bring a unifying approach to connectionist and logic-based principles for visual processing and abstract reasoning respectively. This paper presents a complete neuro-symbolic method for processing images into objects, learning relations and logical rules in an end-to-end fashion. The main contribution is a di erentiable layer in a deep learning architecture from which symbolic relations and rules can be extracted by pruning and thresholding. We evaluate our model using two datasets: subgraph isomorphism task for symbolic rule learning and an image classi cation domain with compound relations for learning objects, relations and rules. We demonstrate that our model scales beyond stateof-the-art symbolic learners and outperforms deep relational neural network architectures.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;neuro-symbolic reasoning</kwd>
        <kwd>end-to-end learning</kwd>
        <kwd>relational representations</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        input such as images [
        <xref ref-type="bibr" rid="ref13">21</xref>
        ]. Our approach provides a new perspective on existing feed-forward
neural networks to build an end-to-end architecture that can not only handle low-level input
but also converge to symbolic relations and rules.
      </p>
      <p>
        Hence, in this paper we focus on the question of whether an end-to-end neural network can
learn objects, relations and rule-based reasoning going from pixels in an image all the way to
logic-based rules. Inspired by the perceptron algorithm [
        <xref ref-type="bibr" rid="ref37 ref38">61, 62</xref>
        ], we de ne an inductive bias to
an otherwise regular feed-forward neural network in order to enable end-to-end neuro-symbolic
learning. The main contribution of this paper is a novel way of constraining the bias of a linear
layer to obtain the semantics of AND and OR gates as well as pruning and thresholding to
extract symbolic rules from that layer. We evaluate our approach in a controlled environment
with two synthetic datasets and present the analysis of the learnt objects, relations and rules.
Our implementation using TensorFlow [1] is publicly available at https://github.com/nuric/pix2rule
with the accompanying data and analysis.
      </p>
    </sec>
    <sec id="sec-2">
      <title>2. Semi-symbolic Layer</title>
      <p>
        Our neuro-symbolic architecture makes use of a di erentiable feed-forward layer that behaves
like conjunction or disjunction. Given continuous inputs 1, 2, . . . ,  ∈ [⊥, ⊤] in which
⊥, ⊤ ∈ R denote some real-valued constants for false and true respectively, we would like
to model a layer that can act like conjunction  = ⋀︀  or disjunction  = ⋁︀  where
 ∈ [⊥, ⊤] is the output. If we take ⊥ = 0, ⊤ = 1 and utilise t-norms [30] to implement fuzzy
logic, as the number of inputs  increases, the operation su ers from vanishing gradients and
becomes unviable for upstream layers such as convolutional neural networks (CNN) [
        <xref ref-type="bibr" rid="ref17">26</xref>
        ]. This
phenomenon occurs due to the 1 out of  failure or success characteristic of conjunction and
disjunction respectively. Hence, we are interested in an operation that does not starve gradients
and eventually converges to the desired semantics.
      </p>
      <p>
        Based on the single-layer perceptron [
        <xref ref-type="bibr" rid="ref38">62</xref>
        ], we propose the semi-symbolic layer (SL):
 =  (∑︁  +  ) (1)  =  (max || − ∑︁ ||) (2)
 
where  are the learnable layer weights,  the non-linear activation function and  ∈ [− 1, 1]
the semantic gate selector. In this formulation we set ⊥ = − 1, ⊤ = 1 and  to be the hyperbolic
tangent function (tanh). While Eq. (1) is the standard feed-forward layer, by adjusting the bias 
we can obtain either conjunctive when  = 1 or disjunctive when  = − 1 semantics. Intuitively,
in the conjunctive case, we are looking for a threshold that is at least as small as the sum of the
weights but not any bigger than the maximum weight such that if at least one input is false, the
output will be false too. For a more detailed derivation and an example implementation, please
refer to Appendix A.1. By gradually adjusting  from 0 to either 1 or -1, we can now maintain a
feed-forward layer that does not starve gradients to upstream layers but eventually converges to
the con gured semantics. The sign of the weights indicate whether the input or its negation is
contributing to the output. Thus, logical negation can be regarded as the multiplicative inverse
of the input ¬ = −  which naturally works with the weights of the layer. When there are
no input connections, i.e. ∀ = 0 the output becomes zero and when there is only one input
the bias becomes  = 0 yielding a pass-through, identity gate.
      </p>
      <p>Although for symbolic inputs  ∈ {⊥, ⊤} and  = {1, − 1}, we achieve the proper behaviour,
una</p>
      <p>ry(X,0)
una
ry(X,0)</p>
      <p>X
X
not nullary(1)
binary(X,Y,1)
not binary(X,Y,0)
nullary(1)
binary(X,Y,1)
binary(Y,X,0)</p>
      <p>Y
Y
t :- not nullary(1), unary(X,0),</p>
      <p>
        not binary(X,Y,0), binary(X,Y,1).
unary(Y,1) t :- bniunlalarryy((X1,)Y,,u1n),abryin(aXr,y0()Y,,uXn,0a)r.y(Y,1),
1–43
(a) Sample target rules for the easy di culty of (b) Example images from the relations game dataset
the graph dataset. For each predicate, the last with the top and bottom rows showing correct
argument denotes the relation id. and incorrect cases respectively.
the output of the semi-symbolic layer does not align with existing many-valued logics such
as Kleene or Lukasiewicz logics [
        <xref ref-type="bibr" rid="ref22">46</xref>
        ]. For example, in the conjunctive case when ∀ = 0 the
output will be negative, Eq. (2), implying that the conjunction of unknown inputs is false. In
practice, when the input is a vector of learnt features from an upstream layer, the model has the
chance to resolve unknown inputs if the task requires it. We leave further theoretic analyses of
this layer’s implied logics as future work.
      </p>
      <p>
        Prune &amp; Threshold In order to obtain symbolic formulas, we prune and then threshold the
weights after training. Similar to decision tree pruning methods [23], each weight is set to zero
 = 0 and pruned if the performance has not dropped by a xed  . Then a threshold value
is picked by sweeping over potential values [min ||, max ||] with a similar performance
check to pruning. Each weight is then set to ′ = 6 sign() which gives su cient saturation
tanh(6) ≈ 0.999. Finally, we repeat the pruning step to remove any wrongly ampli ed weights.
We opt for this simple post-training regime rather than regularisation such as l-norms [
        <xref ref-type="bibr" rid="ref17">26</xref>
        ] to
limit the number of modi cations required to train the semi-symbolic layer.
      </p>
    </sec>
    <sec id="sec-3">
      <title>3. Datasets</title>
      <p>Since low-level signals such as images can produce large noisy latent spaces, we need to
understand whether our approach can robustly learn symbolic rules in a scalable fashion.
Hence, we use two synthetic datasets: subgraph set isomorphism for testing the semi-symbolic
layer over symbolic inputs and image classi cation for the full pipeline. For both datasets, the
model is required to predict a true or false label. Samples from each dataset are shown in Fig. 1
with further details and examples in Appendix B. We opt to use synthetic datasets with known
and controlled parameters to avoid any inherent biases that may arise in real-world datasets.</p>
      <p>
        Subgraph set isomorphism requires a model to decide whether any subgraph of a given
graph is isomorphic to a set of other graphs. We choose graph isomorphism as it is combinatorial
in nature and known to be NP-complete [
        <xref ref-type="bibr" rid="ref12">20</xref>
        ] providing a means to gauge scalability with respect
to the search space of possible solutions. Formally, given a graph  and a set of graphs
H = {ℋ1, ℋ2, . . . , ℋ}, determine whether the target condition  ⇐⇒ ∃ℒ,ℒ ⊆  ∧ ℒ ≃ ℋ 
holds where ≃ is graph isomorphism. Given graphs that either satisfy or do not satisfy , the
objective is to learn a H for a xed . This learning task can be rendered as an instance of
Inductive Logic Programming (ILP) [
        <xref ref-type="bibr" rid="ref27">51</xref>
        ] where H is the hypothesis to learn with no background
knowledge. We encode the problem in Answer Set Programming (ASP) [43] using the generic
predicates nullary/1 for global graph properties, unary/2 and binary/3 for self-edges and directed
edges between nodes respectively with target condition  as head of the rule, see Fig. 1a. Since
we are interested in learning H rather than just subgraph isomorphism, recent advancements in
deep graph neural networks [9, 41] are not suitable for extracting what H has been learnt, also
known as the black-box problem [2]. To adjust the di culty, Table 1, we change the number of
nodes | ()|, nullary, unary, binary relations, number of nodes in any ℋ and the maximum
size of |H| ensuring | (ℋ)| ≤ |  ()|. We focus on these parameters to alter the number of
edges to be learnt |(ℋ)| which in return corresponds to the length of the rules. The average
rule length in the medium di culty is well beyond common datasets such as odd or even, family
tree and graph colouring used in existing neuro-symbolic research [
        <xref ref-type="bibr" rid="ref15 ref29 ref40">24, 64, 53</xref>
        ].
      </p>
      <p>Relations Game dataset consists of an input image with di erent shapes and colours
exhibiting compound relations between them. It has been introduced to evaluate object-based
relational learning in deep neural networks [65]. Existing methods fail to provide a coherent
object, relation and symbolic rule learning in an end-to-end fashion. Along with the four tasks
in Fig. 1b, we create an All multi-task setting and provide the task id as additional input. While
the training set contains pentominoes, shapes with 5 uniformly coloured pixels, the test sets
expand to hexominoes (shapes with 6 pixels) as well as striped shapes with unseen colours. To
ascertain if neuro-symbolic approaches could be more data e cient, we take 100, 1k and 5k
for training and 1k for validation and test splits. We also apply standard data augmentation
during training with random horizontal or vertical ips with an added input noise drawn from
 (0, 0.01). For further details and more examples, please refer to Appendix B.2.</p>
    </sec>
    <sec id="sec-4">
      <title>4. Experiments</title>
      <p>We evaluate our approach’s ability to learn symbolic rules in a di erentiable manner, scalability
and performance in the presence of input noise. To learn full logic rules such as in Fig. 1a, we
stack two semi-symbolic layers, one conjunctive and one disjunctive, for a disjunctive normal
form (DNF) layer. While theoretically di erent combinations of SL layers can extend the space
of possible logic programs to a larger language including constraints and predicates with arity
greater than two, we focus on learning rst-order (recursive) normal logic programs without
function symbols and excluding constraints. We also work with the nullary/1, unary/2 and
binary/3 predicates since any logic program can be converted to a program using this signature.
The DNF layer is constructed with a maximum number of variables | (ℋ)| and xed |H| to
learn rst-order rules rather than just propositional formula. When viewed as a fully-connected
graph with variable symbols as the nodes (similar to Fig. 1a), the maximum number of edges</p>
      <p>CNN</p>
      <p>Objects</p>
      <p>Relations
nullary(0).
nullary(3).
...
unary( ,0).
…
binary( ,</p>
      <p>,
|(ℋ)| corresponds to the maximum length (number of body atoms) of the learnable rules. We
gather all combinations (︀ | ()| )︀ to e ectively curate every possible object to variable binding,
| (ℋ)|
shown as Permute step in Fig. 2. This grounding step converts rst-order normal rules in
question into their propositional instantiations over which SL layers from Section 2 are used.
Although this binding is combinatorial, the evaluation of the rule under every binding can be
computed in parallel which bene ts from the increasing computing infrastructure available. In
cases where the desired rules require speci c constants as literal arguments, the instantiations
could be restricted at the permutation step to adjust the hypothesis space accordingly. After the
conjunctive SL, we use the max operator over the instantiations of any existential variables
that may occur in the rules. The DNF layer is used as a standalone model for the subgraph set
isomorphism dataset or in tandem with upstream image processing layers for the relations game
as in Fig. 2. We train all deep models using Adam [36] with a xed learning rate of 0.001 and
the negative log-likelihood loss on an Intel Core i7 CPU and report the median results to avoid
outliers. For all the hyper-parameters and further training details, please refer to Appendix A.6.</p>
      <p>
        Can the DNF layer learn symbolic rules in a scalable, di erentiable manner? We
train a standalone DNF layer on the subgraph set isomorphism dataset for 10k batch updates with
a batch size of 128. The results with continuous weights (DNF) and with pruning, thresholding
(DNF+t) are compared against two state-of-the-art symbolic learners: ILASP [38] (2i as the
recommended version for non-noisy tasks) and the recent more scalable FastLAS [39]. We also
considered state-of-the-art rule mining system AMIE [37] but it does not support negated rules
and uses breath- rst search that does not scale well to long rules. We were only able to run the
symbolic learners for the easy size because they do not report progress or allow checkpointing
making them infeasible for distributed shared computing infrastructure. FastLAS on an isolated
machine did not terminate after 16 hours on the medium di culty. In Table 2, we observe that
DNF layer scales better than symbolic learners to larger rule sizes and since it is trained for xed
number of iterations maintains a steady training time. Since DNF+t is thresholded, we have
exact logic semantics which correspond to the symbolic rules learnt by the symbolic systems.
We can convert the thresholded weights into ASP rules and test them using clingo [
        <xref ref-type="bibr" rid="ref19">25</xref>
        ] to verify
that our approach has indeed learnt correct symbolic rules in a di erentiable manner.
is less than 0.09 for all entries.
      </p>
      <sec id="sec-4-1">
        <title>Di iculty Easy</title>
      </sec>
      <sec id="sec-4-2">
        <title>Noise</title>
        <p>DNF
DNF+t</p>
        <p>
          Medium
subgraph set isomorphism dataset by randomly ipping the truth values of () in the training
set with a xed probability shown in Table 3. We observe that the DNF layer performs well up
to 0.3 where the median accuracy drops below 0.9. The pruning and thresholding steps seem
to improve the performance with lower levels of noise, likely because incorrect weights are
removed against a non-noisy validation set.
upstream convolutional neural networks (CNN) and object selection layers to construct the full
neuro-symbolic DNF model (DNF), Fig. 2. The image is processed in patches by the CNN to
obtain a set of 9 candidate objects. Since reasoning with 9 objects at the same time creates a
large grounding for the DNF layer, we use an attention based object selection layer. Attention
0.56
0.62
0.50
0.69
0.63
0.57
0.57
0.62
0.49
0.69
0.65
0.60
0.57
0.57
0.49
0.63
0.64
0.54
0.99
0.99
0.79
0.99
0.99
0.95
0.96
0.95
0.81
0.98
0.96
0.95
0.97
0.97
0.84
0.96
0.95
0.94
models [
          <xref ref-type="bibr" rid="ref8">8, 16</xref>
          ] allow neural networks to focus on speci c parts of the input, in this case
selecting relevant patches of the given input image. The attention map is used as the parameters
of a Gumbel-Softmax [33], also known as Concrete [
          <xref ref-type="bibr" rid="ref21">45</xref>
          ], distribution to gradually sharpen
the attention maps and have a clear correspondence between the selected objects and the
rules applied thereafter. We leave more complex, unsupervised scene object recognition and
decomposition methods [
          <xref ref-type="bibr" rid="ref20 ref6">14, 44, 10</xref>
          ] for future work. We then use a single feed-forward layer
with shared weights to compute all relations between the selected objects, and pass onto a DNF
layer. For further details please refer to Appendix A. We construct 2 additional variants: DNF-h
has extra hidden DNF layers, one for each 14 invented predicates, and DNF-r iterates the DNF
layer twice learning recursive rules with 7 invented predicates. The invented predicates are
evaluated in parallel using matrix operations similar to a feed-forward layer with many outputs.
        </p>
        <p>
          Is the DNF model more data e cient? We compare our approach against PrediNet [65],
an explicitly relational neural network and the current state-of-the-art in the relations game
dataset. Note that PrediNet already outperforms MLP baselines and Relation Networks [
          <xref ref-type="bibr" rid="ref41">63</xref>
          ].
Looking at columns with di erent data sizes in Table 4, we observe that the DNF models
consistently match or outperform PrediNet, especially DNF-h with training size 100 for the
between task. This gap eventually narrows as the training size is increased. Despite failing
to recognise and reason with four objects in the Occurs and XOccurs tasks with 100 training
examples, all the models seem to tackle them better when mixed with other tasks, i.e. the All
task. This might suggest that a form a curriculum learning [11] is bene cial to learn and reason
with 2 objects, then 3 and nally 4 objects. The failure cases observed with 100 training data
points highlight how our approach is still prone to over- tting due to its neural network based
formulation which may be further mitigated using standard techniques such as regularisation.
        </p>
        <p>Can the DNF model generalise to unseen shapes and colours? After training on
pentominoes, the models are tested on unseen hexominoes and striped shapes, Appendix B.2. The
results for Hex. and Stripe in Table 4 are similar to that of pentominoes for all models. This lack
of change might be because of the tasks’ requirement to determine whether the shapes match
or not, which can encourage a simple subtraction based representation that would generalise to
unseen shapes and colours. Indeed, PrediNet uses subtraction as the main relational operator
while our model would need to learn that solely from examples.</p>
        <p>Does image reconstruction improve DNF model performance? We try to reconstruct
the image from the selected objects using deconvolutional layers as an auxiliary loss (DNF-hi),
see Appendix A.5 for reconstruction details and Appendix D for examples. Comparing DNF-hi
and DNF-h rows, we do not observe any improvement above 5% despite the extra computation
required to reconstruct the images. Although the representations of objects would need to
accommodate both the reasoning and the reconstruction, we believe the simplicity of the shapes
might be why the auxiliary loss does not yield any advantage for the rule learning.</p>
        <p>Can we extract symbolic rules in an image classi cation task? Finally, the goal is to
obtain a coherent neuro-symbolic pipeline where objects, their relations and symbolic rules
are learnt. We take the best variant DNF-h and threshold its weights DNF-h+t to get symbolic
rules. Comparing DNF-h with DNF-h+t in Table 4, only if DNF-h achieves ≥ 0.99 accuracy
does DNF-h+t consistently yield better than random performance. This highlights the di culty
of learning both the meaning of the predicates as well as the rules in tandem. Yet, there are
some successful outliers ignored by the median, please refer to Appendix D for the full results.
Input
0 1 2 0 1 2 0 1 2 0 1 2
(a) Attention maps learnt during the object se- (b) Object arguments (top and bottom rows) that make
lection phase. Gumbel-Softmax with low binary(X,Y,6) true or false exhibit no common
temperature approximates hard attention. pattern of the principal concepts of the dataset.</p>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>5. Analysis</title>
      <p>To understand what the model has learnt and how it is behaving, we take a single successful run
(≥ 0.95 accuracy) from the DNF model with image reconstruction on the between task with
1k training examples. We choose this task because it is smaller in size and the model achieves
0.99 and 0.97 test hexominoes accuracy prior to and after thresholding respectively. For further
examples and analysis, please refer to Appendix D.</p>
      <p>What are the selected objects? To ensure whether the relations and rules actually work
with the desired objects, we plot the attention maps obtained from the object selection step. As
shown in Fig. 3a, the model learns to assign higher scores to patches with objects and iteratively
selects them. Due to the random sampling, the order of the selected objects may vary.</p>
      <p>In a successful run, what are the rules learnt? Since the thresholded model has a
oneto-one correspondence with the desired logic semantics, we extract out the rule used to solve
the between task, e.g. input images shown in Fig. 3a:
between :- unary(X,4), not unary(Y,6), binary(Y,Z,6), not binary(Y,Z,9),</p>
      <p>not binary(Z,Y,3), binary(Z,Y,5), not binary(Z,Y,9), binary(Z,Y,14).</p>
      <p>which achieves 0.97 accuracy on the test hexominoes set. This is the rst result we are aware
of that presents di erentiable rule learning with learnt predicates on continuous
representations of images in an end-to-end fashion. To further validate this rule, we threshold &gt; 0 the
interpretation, relations box in Fig. 2, and pass them, along with the rule to clingo. Over a
sample test batch of 64 examples, clingo solves 91% of them verifying the correctness of the
learnt rules by the DNF layer. This seamless integration of symbolic relations and rules with
distributed representations of objects re ect on how the human brain might be using symbolic
and sub-symbolic representations in tandem [35].</p>
      <p>What do the learnt predicates mean? We iteratively remove one atom from the body
and check the drop in accuracy over the same sample test batch. Removing binary(X,Y,6)
yields the highest drop of 18%. Hence, we plot cases of binary(X,Y,6) in Fig. 3b but fail to
notice any alignment between the primary concepts of the dataset such as shape, colour or
position. Although at rst this may seem odd, since there is no regularisation for any sort of
disentanglement, we would not expect or assume any correlation between the concepts and the
learnt relations. This is in contrast with explainable AI [2] and the promise of interpretability
using neuro-symbolic methods which is fundamentally limited by our ability to decode learnt
representations. On the other hand, this phenomenon does not arise when the input predicates
are known and xed such as in the graph isomorphism dataset where the learnt rules have
interpretable meanings in the form of graphs, see Fig. 1a.</p>
    </sec>
    <sec id="sec-6">
      <title>6. Related Work</title>
      <p>
        Neuro-symbolic [
        <xref ref-type="bibr" rid="ref5">13</xref>
        ] architectures and methods have a proli c history. We can categorise related
work based on how neural or symbolic they are. On the far neural end of the spectrum, we
can consider deep neural networks designed to perform symbolic manipulation such as Neural
Turing Machines [28] and its successor the Di erentiable Neural Computer [29]. They attempt
to create universal computers that can be trained and, in theory, perform symbol manipulation
for example by solving algorithmic tasks. However, how they perform symbolic representation
and manipulation is not clear. Studies of algorithmic tasks using recurrent neural network based
controllers suggest limitations in the extent of symbolic manipulation by continuous dense
representations [74]. This symbolic manipulation can also extend to program execution with
Neural-program interpreters [
        <xref ref-type="bibr" rid="ref33">57</xref>
        ] and Neural Symbolic Machines [42] which operate in discrete
steps but use reinforcement learning in doing so. Discrete data-structures such as stacks have
also been used in aid of this problem such as Stack-augmented Recurrent Nets [34].
      </p>
      <p>
        Going one step closer to symbolic, architectural biases are often added to constrain and
re ne the behaviour of neural networks. For example, Relation Networks [
        <xref ref-type="bibr" rid="ref41">63</xref>
        ] create a pairwise
object representation bottleneck and demonstrate improvements in visual question answering
tasks. This architectural inductive bias can be traced to many models all the way and including
PrediNet [65]. More recently, an application of rule based systems that also use
GumbelSoftmax [33] to sharpen the rule selection was proposed with Neural Production Systems [27].
Unsupervised deep representation learning using auto-encoders were also shown to work in the
planning domain where the latent representation was converted to propositional atoms [5, 6]. In
relation to the subgraph isomorphism problem, one can also deploy graph neural networks [9]
that utilise message passing between nodes. In particular, Graph Matching Networks [41] are
designed to determine graph similarities but it remains unknown how the matched graphs
could be extracted from continuous weights.
      </p>
      <p>
        Methods described so far do not consider logical formalisms or logic programs. DeepLogic [
        <xref ref-type="bibr" rid="ref10">18</xref>
        ]
provides a rst insight into attempts of learning symbolic reasoning in an end-to-end fashion
using memory networks [
        <xref ref-type="bibr" rid="ref45 ref47">71, 69</xref>
        ] which has gated recurrent-units (GRU) [
        <xref ref-type="bibr" rid="ref9">17</xref>
        ] as the basis of
symbolic manipulation. Neural Logic Machines [
        <xref ref-type="bibr" rid="ref14">22</xref>
        ] use principles of forward-chaining [
        <xref ref-type="bibr" rid="ref38">62</xref>
        ]
with nullary, unary and binary predicates implemented as MLPs with input permutations to
tackle algorithmic tasks. However, due to the use of MLPs, the learnt rules or reasoning steps
cannot be symbolically extracted. Our approach falls in this section of the spectrum: constrain or
make neural networks exhibit behaviours of logical reasoning. That is, to build an architecture
with the right biases in order to learn and leverage structured logical reasoning in the form of
objects, relations and rules.
      </p>
      <p>
        Going further away from deep neural networks, logic based architectures become more
prominent. For example, Lifted Relational Neural Networks [
        <xref ref-type="bibr" rid="ref44">68</xref>
        ] use rule sets as templates for
constructing neural networks, i.e. the connection paths. Logic Tensor Networks [
        <xref ref-type="bibr" rid="ref40">64</xref>
        ] use dense
embeddings of constants while also constructing a deductive neural network. TensorLog [
        <xref ref-type="bibr" rid="ref11">19</xref>
        ]
similarly builds factor graphs which in return yield the neural network architecture. These
approaches have similarities to the more recent Logical Neural Networks [
        <xref ref-type="bibr" rid="ref34">58</xref>
        ] which assemble
neural networks from logical formulae and constrain the weights to achieve conjunction and
disjunction semantics. Our more exible approach is not bound to a xed logic program and
does not require any constraints on the weights whilst also handling negation.
      </p>
      <p>
        One could also use distributed representations for constants or predicates. For example
similarity between vectors could provide basis for a logical calculus [
        <xref ref-type="bibr" rid="ref36 ref50">60</xref>
        ]. Neural Theorem
Provers [
        <xref ref-type="bibr" rid="ref35">59</xref>
        ] learn embeddings of predicates by unrolling given logic programs using
backwardchaining [
        <xref ref-type="bibr" rid="ref3">4</xref>
        ], e ectively following the steps of symbolic reasoning prior to any learning. These
works have been inspired by word embeddings [
        <xref ref-type="bibr" rid="ref26 ref31 ref7">50, 55, 15</xref>
        ] that capture distributional semantics
of words. More recently, Neural Datalog [
        <xref ref-type="bibr" rid="ref25">49</xref>
        ] uses vector representations along with a datalog
program to improve performance of temporal modelling.
      </p>
      <p>
        Closer to the symbolic end of the spectrum, one could also attempt to decompose or parse
continuous input completely into symbolic entities or concepts and then perform symbolic
reasoning. Neural scene parsing as done in Neuro-Symbolic VQA [
        <xref ref-type="bibr" rid="ref49">73</xref>
        ] and following works,
completely decompose images into objects, their properties before performing visual question
answering. For synthetic datasets where the decomposition is successful, it outperforms any
end-to-end approach. We consider scene parsing closer to symbolic approaches since the
reasoning about objects and their relations is not done by neural networks but by the manually
engineered reinforcement learning environment with pre-de ned functions. Neuro-symbolic
Concept Learner [
        <xref ref-type="bibr" rid="ref24">48</xref>
        ] extends scene decomposition by learning concepts given in a parsed
question such as red in a joint fashion using reinforcement learning in which the environment
is a symbolic program executor.
      </p>
      <p>
        Moving away from neural networks but attempting to harness gradient descent, we have
approaches such as  -ILP [
        <xref ref-type="bibr" rid="ref15">24</xref>
        ] and derivations [66] which implement t-norms to create di
erentiable logic programs to nd a suitable hypothesis in an ILP setting. More recently, by directly
modelling rule membership of atoms as learnable weights, Neural Logic Networks [
        <xref ref-type="bibr" rid="ref29 ref30">54, 53</xref>
        ]
provide a competitive di erentiable ILP system that leverages gradient descent. The in uence of
gradient descent can also be found in full symbolic reasoning systems like DeepProblog [
        <xref ref-type="bibr" rid="ref23">47</xref>
        ] and
NeurASP [
        <xref ref-type="bibr" rid="ref48">72</xref>
        ] which attempt to propagate gradients through logic programs in order to train
input neural networks such as CNNs that recognise hand-written digits and perform addition
of the recognised digits. This resembles continuous formulations of logic such as di erentiable
stable and supported semantics that utilise matrix multiplications and non-linear activations to
realise logic operations [7].
      </p>
    </sec>
    <sec id="sec-7">
      <title>7. Conclusion</title>
      <p>We presented a uni ed neuro-symbolic framework for learning objects, their relations and
symbolic rules in an end-to-end fashion using semi-symbolic layers. Evaluation on two datasets
portray competitive if not better results of our approach against symbolic learners and deep
neural networks. Since symbolic rules can be extracted and examined, we plan to apply this
technique to more decision making critical domains such as reinforcement learning.</p>
    </sec>
    <sec id="sec-8">
      <title>Acknowledgments</title>
      <p>We would like to thank Murray Shanahan for his helpful comments. We would also like to
thank Mark Law for his support in running ILASP and FastLAS symbolic learners.</p>
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abadi.pdf.</p>
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We use two models in our experiments that share the common DNF layer described in Section 2
and Section 4. In this section, we provide details on the semi-symbolic layer as well as the larger
model used for the image classi cation task.</p>
      <sec id="sec-8-1">
        <title>A.1. Semi-symbolic Layer</title>
        <p>
          The semi-symbolic layer builds on the semantics of how a regular single-layer perceptron [
          <xref ref-type="bibr" rid="ref38">62</xref>
          ]
behaves in order to achieve AND semantics. Let’s start with the formulation for the
preactivation value for a single-layer perceptron:
        </p>
        <p>Now, suppose we are interested in obtaining AND gate semantics with a pre-activation value
 if all inputs are true and −  if one of the inputs are false. To simplify the derivation, suppose
the magnitudes of the weights are equal ∀  =  :</p>
        <p>A. Model Details</p>
        <p>where a true input means  = 1.0 if the weight is positive or  = − 1.0 otherwise. We use
the absolute value of the weights since the sum of the weights multiplied by matching inputs
would yield the sum of the absolute value of the weights. Solving the above system of equations
for  , we obtain:
∑︁  +  = 

∑︁ || +  =</p>
        <p>= max || −
∑︁ ||

Since the weights would not be equal during training, we replace the rst term to obtain the
nal equation Eq. (2) presented in Section 2:
which preserves the desired AND gate semantics. The derivation for the disjunctive case is
identical and leads to the bias terms ipped, i.e. sum - max. A fully functional implementation
of the semi-symbolic layer in TensorFlow can be found in Listing 1.
1–43
(3)
(4)
(5)
(6)
(7)
(8)
To process the input images in the relations game dataset, Section 3, we use a single layer
convolutional neural network (CNN) with a kernel size of 4x4, stride 4, relu activation and 32
lters. We use these options to keep the image processing layer simple and less computationally
intensive. The input to the layer is a single relations game image, 12x12x3 and the output is
3x3x32. Note that, with this con guration the CNN is aligned to the grid cells of the image in
which the objects are contained.</p>
        <p>For each output of the CNN, 3x3x32, we also append the location information yielding a
nal result of 3x3x36. The coordinates are computed on a linear scale from 0 to 1 based on the
location of the patch in the 3x3 grid. The representations are attened into 9x36 and passed
onto the selection layer. Example representations learnt by this layer are shown in Table 5.</p>
      </sec>
      <sec id="sec-8-2">
        <title>A.3. Object Selection</title>
        <p>
          Given a set of objects with continuous representations in the form of a matrix  ∈ R×  where
 is the number of objects and  is the number of dimensions, the object selection layer selects
 many relevant objects. The number of objects  is xed and determined by the task. We use
2 for Same, 3 for Between and 4 for the rest of the tasks including All. The relevance, or score,
is learnt using single feed-forward layer. Concretely, the 9x36 object matrix obtained from the
CNN layer, is mapped to a vector  ∈ R9. Then these unnormalised scores are used as logits for
a Gumbel-Softmax [33] or Concrete [
          <xref ref-type="bibr" rid="ref21">45</xref>
          ] distribution, from which we sample once to obtain an
attention map  ∈ R. We then use an iterative score inversion principle:
        </p>
        <p>+1 = ( − ) + (1 − )
where  = 100 is the inversion constant. We iterate for  times selecting one object at each
iteration and by inverting its score, the layer learns to select  distinct objects.</p>
        <p>Similar to previous work that use Gumbel-Softmax as a means of di erentiable categorical
distribution [5], we anneal the temperature of the distribution starting from 0.5, down to 0.01
with an exponential rate of 0.9 every step after 20 epochs. That allows the model to gradually
learn and then sharpen the object selection process. At a temperature of 0.01, the attention
maps become one-hot vectors allowing a clear correspondence between the selected objects and
the applied rules. This is in contrast with soft attention [8] mechanisms such as the dot-product
attention used in PrediNet which allow continuous selection of image patches as a single object.</p>
      </sec>
      <sec id="sec-8-3">
        <title>A.4. Object Relations</title>
        <p>Once the objects are selected, we compute all unary and binary relations between them using
their continuous representations. We use a single feed-forward layer for unary and another for
binary relations:</p>
        <p>
          unary(, ) = tanh( + )
binary(, , ) = tanh( [, ,  −  ] +  )
where [] is the concatenation operator and  ,  are di erent weights for each equation. The
result can be considered a fully connected graph with the selected objects as nodes, and the
computed unary and binary relations as edges. We opt for this single-layer formulation to
keep it computationally less intensive and focus on rule learning. We leave more complex and
expressive relational layers that compute interactions between dense entity vectors such as
Neural Tensor Networks [
          <xref ref-type="bibr" rid="ref43">67</xref>
          ] as future work.
        </p>
      </sec>
      <sec id="sec-8-4">
        <title>A.5. Image Reconstruction</title>
        <p>To test the hypothesis whether image reconstruction can help with di erentiable rule learning,
we provide an auxiliary loss by reconstructing the full input image from the selected objects.
Starting from a matrix of selected objects  ∈ R× , we spatially broadcast them into a 3x3
grid. In the speci c case of relations game dataset, the selected objects are broadcast into a
tensor of shape mx3x3x36. We then use 2 deconvolution layers, also known as transposed
convolution, with 32 lters, kernel size 5, relu activation and stride 2 to expand the tensor to
shape mx12x12x32. For the nal layer, we apply another deconvolution with 4 lters, kernel
size 5 and a stride of 1 yielding mx12x12x4. The rst three colour channels are combined with
the last masking channel to obtain the nal output. The reconstructed image is trained using
mean squared error and example reconstructions can be seen in Fig. 19 and in Fig. 22.
1–43</p>
        <p>(9)
(10)
(11)</p>
        <p>Key
# batch updates Rels. Game</p>
        <p># batch updates Graph
# batch updates per eval</p>
        <p>learning rate
batch size Rels. Game
batch size Graph
# of repeated runs</p>
        <sec id="sec-8-4-1">
          <title>Input noise stddev Rels. Game rng seed Rels. Game rng seeds Graph Value</title>
          <p>There are three main categories of hyperparameters used in this work: model, dataset and
training. We cover all of them in Table 6 and Table 7. The magnitude of the semantic gate
selector | | presented in Section 2 is gradually adjusted during training according to a xed
exponential schedule. For the image classi er model, we start with 0.01 and increase to 1.0 with
an exponential rate of 1.1 while for the subgraph set isomorphism dataset we start higher 0.1
and use the same rate of 1.1. We start the gate at a higher value for the graph isomorphism
dataset since the input is already symbolic, i.e. ∀ ∈ {− 1, 1}. The sign of  is pre-determined
by the layer type, conjunctive or disjunctive within a DNF layer, see Sections 2 and 4.</p>
        </sec>
      </sec>
    </sec>
    <sec id="sec-9">
      <title>B. Dataset Details</title>
      <p>
        We use two synthetic datasets to create a controlled environment in which we can evaluate
our approach. The rst dataset is based on the subgraph isomorphism problem and is used
as the basis for an unbiased Inductive Logic Programming (ILP) [
        <xref ref-type="bibr" rid="ref27">51</xref>
        ] task. The second dataset
is an image classi cation task which involves objects of di erent shapes and colour in some
compound relation such as between two identical objects.
      </p>
      <sec id="sec-9-1">
        <title>B.1. Subgraph Set Isomorphism</title>
        <p>
          The subgraph set isomorphism task is designed to create an unbiased combinatorial search
space for rule learning. To generate data points, we uniformly sample unique graphs for H
and then sample  of which subgraphs are isomorphic. The resulting examples are checked
using the answer set solver clingo [
          <xref ref-type="bibr" rid="ref19">25</xref>
          ]. In total we sample 10k graphs and take only the unique
ones to ensure any partitions of the data are disjoint. We then split into training, validation and
test with sizes 2k, 1k and 1k respectively. Since each relation could be positively, negatively or
        </p>
        <sec id="sec-9-1-1">
          <title>Number of variables that can appear in the rules learnt by the hidden DNF layer of the DNF-h models.</title>
        </sec>
        <sec id="sec-9-1-2">
          <title>Maximum number of rule definitions per predicate in the hidden layer.</title>
        </sec>
        <sec id="sec-9-1-3">
          <title>The number of predicates learnt by the hidden DNF layer. Specif</title>
          <p>ically, 2 nullary, 4 unary and 8 binary predicates.</p>
        </sec>
        <sec id="sec-9-1-4">
          <title>The target label can be defined at most by 8 di erent rules.</title>
        </sec>
        <sec id="sec-9-1-5">
          <title>The number of extra predicates learnt by the DNF layer in the recursive configuration DNF-r. Specifically, 1 nullary, 2 unary and 4 binary predicates.</title>
        </sec>
        <sec id="sec-9-1-6">
          <title>Maximum number of rule definitions per predicate including the label in the recursive DNF layer, DNF-r.</title>
          <p>be absent in the rule, the upper bound on the search space for possible rules is 3|(ℋ)|. This
ensures there is no bias in the rules or heuristics that can be used to prune the search space. As
a result, any atom can appear in the rule equally likely.</p>
          <p>
            An example set of rules H from the medium dataset size is shown in Table 8. The head of
the rules  corresponds to the desired target label for a given set of context facts, i.e. whether
the given graph is subgraph isomorphic to the set of graphs represented by the rules. The
symbolic learners timeout on this size due to the long rules. Rules of this length are uncommon
in many standard ILP datasets [
            <xref ref-type="bibr" rid="ref15">24</xref>
            ] used to evaluate a learner. In order for the rules to be safe in
ASP, that is every variable appears in at least one positive atom, we add obj(...) predicate for
every variable that is true for every grounded object. We also add the uniqueness of variables
nullary(0), nullary(2), not nullary(3), unary(V0,0), unary(V0,1), not unary(V0,2),
not unary(V0,3), not unary(V1,0), not unary(V1,1), not unary(V1,2), not unary(V1,3),
unary(V2,0), unary(V2,2), binary(V0,V1,0), binary(V0,V1,1), binary(V0,V1,2),
binary(V0,V1,3), not binary(V0,V1,4), not binary(V0,V1,5), not binary(V0,V2,0),
binary(V0,V2,2), not binary(V0,V2,3), not binary(V0,V2,5), binary(V1,V0,1),
not binary(V1,V0,2), not binary(V1,V0,3), not binary(V1,V0,4), not binary(V1,V0,5),
binary(V1,V2,0), binary(V1,V2,3), not binary(V1,V2,4), binary(V2,V0,0),
not binary(V2,V0,2), binary(V2,V0,3), not binary(V2,V0,4), not binary(V2,V1,1),
not binary(V2,V1,3), not binary(V2,V1,4), not binary(V2,V1,5), obj(V2), V2 != V0,
V2 != V1, obj(V0), V0 != V1, obj(V1).
nullary(0), not nullary(1), not nullary(2), unary(V0,0), unary(V0,1), unary(V0,3),
unary(V0,4), not unary(V1,0), not unary(V1,2), not unary(V1,4), not unary(V2,0),
not unary(V2,1), unary(V2,2), not unary(V2,3), not unary(V2,4), not binary(V0,V1,2),
binary(V0,V1,3), binary(V0,V1,4), binary(V0,V1,5), not binary(V0,V2,1), binary(V0,V2,2),
binary(V0,V2,3), not binary(V0,V2,4), binary(V0,V2,5), not binary(V1,V0,2),
binary(V1,V0,3), not binary(V1,V0,4), not binary(V1,V0,5), not binary(V1,V2,4),
not binary(V1,V2,5), not binary(V2,V0,0), not binary(V2,V0,1), not binary(V2,V0,2),
binary(V2,V0,4), not binary(V2,V1,0), not binary(V2,V1,1), not binary(V2,V1,4),
obj(V2), V2 != V0, V2 != V1, obj(V0), V0 != V1, obj(V1).
constraint by encoding  ̸=  if  ̸=  since in graph isomorphism every node can at most
be mapped to one other node, i.e. a one-to-one mapping is required. This is only done when
evaluating with clingo [
            <xref ref-type="bibr" rid="ref19">25</xref>
            ] or using symbolic learners.
          </p>
        </sec>
      </sec>
      <sec id="sec-9-2">
        <title>B.2. Relations Game</title>
        <p>The relations game dataset consists of an input image with a desired binary label. There are
3 sets: pentominoes, hexominoes and stripes, examples of which are shown in Fig. 4. The
original dataset presented in PrediNet [65] comes with 250k examples per set and with a higher
resolution of 36x36x3. At the high resolution, each block of a shape consists of 3x3 pixels and
since this is redundant, we convert each block to a single pixel reducing the image without any
loss of information to 12x12x3. This conversion reduces the computational power required to
run the experiments without changing the tasks.</p>
        <p>During training we apply standard data augmentation: (i) random horizontal or vertical ips,
(ii) random 90 degrees counter-clockwise rotation and (iii) added input noise drawn from a
normal distribution with 0.01 as the standard deviation. This augmentation is applied to every
batch and the randomness is drawn from a random number generator with a xed seed of 42.</p>
      </sec>
    </sec>
    <sec id="sec-10">
      <title>C. Training Details</title>
      <p>We train all deep models using the Adam [36] optimiser with a learning rate of 0.001. The
models are trained for a xed number of batch updates and evaluated every 200 batch updates.
We use the negative log-likelihood as the loss function for the binary predictions. For all the
hyperparameters used in training, please refer to Table 6. The models are trained on a shared
pool of computers all having Intel Core i7 CPUs. Note that the run times of the deep models
may vary depending on the shared workload on the worker machine. Once the training is
complete for the DNF models, we prune and threshold the weights to obtain DNF+t variants, as
described in Section 2.</p>
      <p>The training curves for every deep model trained on every dataset and hyperparameter
con guration, can be found in Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11 and Fig. 12. For
relations game training curves, the slight dip in accuracy around epoch 70 when trained with
only 100 examples corresponds to the point at which the semantic gate | | becomes close to 1.
This indicates that if the model is over tting, it almost has to relearn the task with the desired
semantics since this phenomenon is less pronounced with more training examples and almost
absent in the graph dataset. We also observe mode collapses in the recursive con guration
of the DNF model (DNF-r) in which the accuracy suddenly drops when the semantic gate | |
becomes closer to 1 around epoch 70. We believe this is due to the recursion in the model
making a recovery to the desired disjunctive normal form semantics more di cult. The DNF-r
either performs well with some runs surviving the saturation | | = 1.0 and some collapsing.</p>
    </sec>
    <sec id="sec-11">
      <title>D. Further Results</title>
      <p>This section includes further tables and gures to complement and extend the main content
presented in the paper. The description for each gure is provided in the caption.
Same</p>
      <p>Between</p>
      <p>Occurs</p>
      <p>XOccurs
Same</p>
      <p>Between</p>
      <p>Occurs</p>
      <p>XOccurs
(c) Example striped shapes where each object has 9 pixels with a striped colour pattern.</p>
      <p>1.0
0.9
e0.8
vu
l
a
0.7
0.6
0.5
1.0
variable
train_acc
validation_acc
test_acc
20 epoch 30
20 epoch 30
0
10
40
20 epoch 30
60 epoc8h0
variable
train_acc
test_hexos_acc
test_pentos_acc
validation_acc
test_stripes_acc
task = xoccurs | train_size = 100
task = xoccurs | train_size = 1000
task = xoccurs | train_size = 5000
60 epoc8h0
task = same | train_size = 100
task = al | train_size = 100
variable
train_acc
test_hexos_acc
test_pentos_acc
validation_acc
test_stripes_acc
task = occurs | train_size = 100
task = occurs | train_size = 1000
task = occurs | train_size = 5000
60 epoc8h0
task = al | train_size = 100
task = occurs | train_size = 100
task = occurs | train_size = 1000
task = occurs | train_size = 5000
task = same | train_size = 100
task = same | train_size = 1000
task = same | train_size = 5000
variable
train_acc
test_hexos_acc
test_pentos_acc
validation_acc
test_stripes_acc
20
40
100
120
140
0
20
40
100
120
140
0
20
40
100
120</p>
      <p>140
1.1
1.0
0.9
le0.8
vau0.7
0.6
0.5
task = al | train_size = 100
task = xoccurs | train_size = 100
task = xoccurs | train_size = 1000</p>
      <p>task = xoccurs | train_size = 5000
task = occurs | train_size = 100
task = occurs | train_size = 1000
task = occurs | train_size = 5000
variable
train_acc
test_hexos_acc
test_pentos_acc
validation_acc
test_stripes_acc
20
40
100
120
140
0
20
40
100
120
140
0
20
40
100
120</p>
      <p>140
task = xoccurs | train_size = 100
task = xoccurs | train_size = 1000
task = xoccurs | train_size = 5000
task = occurs | train_size = 100
task = occurs | train_size = 1000
task = occurs | train_size = 5000
task = al | train_size = 100
task = al | train_size = 1000
task = al | train_size = 5000
variable
train_acc
test_hexos_acc
test_pentos_acc
validation_acc
test_stripes_acc
60 epoc8h0
task = same | train_size = 100
task = same | train_size = 1000
task = same | train_size = 5000
task = al | train_size = 100
task = al | train_size = 1000
task = al | train_size = 5000
task = occurs | train_size = 100
task = occurs | train_size = 1000</p>
      <p>task = occurs | train_size = 5000
20
40
100
120
140
0
20
40
100
120
140
0
20
40
100
120</p>
      <p>140
1.0
0.9
0.6
0.5
1.0
0.9
0.6
0.5
1.0
0.9
0.5
noise = 0.15 | difficulty = hard
noise = 0.15 | difficulty = medium
noise = 0.15 | difficulty = easy
noise = 0.3 | difficulty = hard
noise = 0.3 | difficulty = medium
noise = 0.3 | difficulty = easy
DNF</p>
      <p>DNF+t
model</p>
      <p>DNF</p>
      <p>DNF+t
model</p>
      <p>DNF</p>
      <p>DNF+t
model
1.00
0.95
0.90
cc0.85
t_sae
t0.80
0.75
0.70
1.00
0.95
0.90
cc0.85
t_sae
t0.80
0.75
0.70
1.00
0.95
0.90
cc0.85
t_sae
t0.80
0.75
0.70
noise = 0.0 | difficulty = easy
noise = 0.0 | difficulty = hard
noise = 0.0 | difficulty = medium
noise = 0.15 | difficulty = easy
noise = 0.15 | difficulty = hard
noise = 0.15 | difficulty = medium
noise = 0.3 | difficulty = easy
noise = 0.3 | difficulty = hard
noise = 0.3 | difficulty = medium
DNF</p>
      <p>DNF+t</p>
      <p>DNF</p>
      <p>DNF+t</p>
      <p>DNF</p>
      <p>DNF+t
difficulty = easy
difficulty = hard
difficulty = medium
fastlas
ilasp</p>
      <p>DNF</p>
      <p>DNF+t
fastlas
ilasp</p>
      <p>DNF</p>
      <p>DNF+t
fastlas
ilasp</p>
      <p>DNF</p>
      <p>DNF+t
model
model
model
train_size = 5000 | task = xoccurs
train_size = 5000 | task = same
train_size = 5000 | task = between
train_size = 5000 | task = al
train_size = 5000 | task = occurs
variable
train_acc
test_hexos_acc
test_pentos_acc
validation_acc
test_stripes_acc</p>
      <p>DNF-hDNF-hiDNF DNF-iDNF-riDNF-PmrreoddiNeDelNtF-hD+NtF-r+DtNF+DtNF-hDi+NtF-ri+DNtF-i+t DNF-hDNF-hiDNF DNF-iDNF-riDNF-PmrreoddiNeDelNtF-hD+NtF-r+DtNF+DtNF-hDi+NtF-ri+DNtF-i+t DNF-hDNF-hiDNF DNF-iDNF-riDNF-PmrreoddiNeDelNtF-hD+NtF-r+DtNF+DtNF-hDi+NtF-ri+DNtF-i+t DNF-hDNF-hiDNF DNF-iDNF-riDNF-PmrreoddiNeDelNtF-hD+NtF-r+DtNF+DtNF-hDi+NtF-ri+DNtF-i+t DNF-hDNF-hiDNF DNF-iDNF-riDNF-PmrreoddiNeDelNtF-hD+NtF-r+DtNF+DtNF-hDi+NtF-ri+DNtF-i+t
xoccurs
100
0.01
0.17
0.01
0.11
0.00
0.04
0.01
0.17
0.03
0.13
0.00
0.00
0.00
0.02
0.16
0.02
0.10
0.00
0.05
0.01
0.18
0.04
0.13
0.01
0.00
0.00
0.01
0.17
0.01
0.12
0.02
0.03
0.02
0.17
0.03
0.14
0.01
0.00
0.03</p>
      <p>Hex.
loss along with median absolute deviation. Only for XOccurs task do we see an improvement.
all
between
occurs
same
xoccurs
100
1000
5000
100
1000
5000
100
1000
5000
100
1000
5000
100
1000
5000</p>
      <sec id="sec-11-1">
        <title>Hexos</title>
        <p>False</p>
        <p>True
0.94± 0.06
2
2
2
2
0
1
1
1
Input
0
1
0.996
0.991
0.942
0.938
Input
Object 1
1
1
1
1
1
0
1
1
1
Input
2
1
binary(X,Y,23) - 1.0
unary(V0,10)
:c3unary(V0,10)
:unary(V0,11)
:c1unary(V0,11)
:binary(V0,V1,23)
:binary(V0,V1,23)
:c3binary(V0,V1,23)
:Input</p>
      </sec>
    </sec>
  </body>
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