=Paper=
{{Paper
|id=Vol-432/paper-7
|storemode=property
|title=ACE View --- an Ontology and Rule Editor based on Attempto Controlled English
|pdfUrl=https://ceur-ws.org/Vol-432/owled2008eu_submission_11.pdf
|volume=Vol-432
|dblpUrl=https://dblp.org/rec/conf/owled/Kaljurand08
}}
==ACE View --- an Ontology and Rule Editor based on Attempto Controlled English==
https://ceur-ws.org/Vol-432/owled2008eu_submission_11.pdf
ACE View — an ontology and rule editor
based on Attempto Controlled English
Kaarel Kaljurand
Institute of Computational Linguistics, University of Zurich
kalju@ifi.uzh.ch
Abstract. We describe the architecture of a novel ontology and rule
editor ACE View. The goal of ACE View is to simplify viewing and
editing expressive and syntactically complex OWL/SWRL knowledge
bases by making most of the interaction with the knowledge base happen
via Attempto Controlled English (ACE). This makes ACE View radically
different from current OWL/SWRL editors which are based on formal
logic syntaxes and general purpose graphical user interface widgets. ACE
View integrates two mappings, ACE→OWL/SWRL and OWL→ACE,
and is implemented as a plug-in for Protégé 4.
1 Introduction
We describe the architecture of a novel ontology and rule editor ACE View. The
goal of ACE View is to simplify the exploration and editing of expressive and
syntactically complex OWL 2 [12] ontologies and SWRL [7] rulesets by basing
the user interface on Attempto Controlled English (ACE) [3]. This makes ACE
View radically different from current OWL/SWRL editors which are based on
formal logic syntaxes and general purpose graphical user interface widgets (trees,
checkboxes, etc.), and which are often seen as too complicated and confusing for
domain experts with no background in formal methods [5]. ACE View integrates
two mappings, ACE→OWL/SWRL and OWL→ACE, and is implemented as a
plug-in for Protégé 41 .
The emerging OWL 2 specification describes several serialization syntaxes
for OWL ontologies (RDF and XML based, functional-style, Manchester OWL
Syntax). Some of these syntaxes are oriented towards machines and are thus
inherently difficult to read and write for humans. Others have been designed for
logicians and programmers, but lack the features that would bring OWL closer
to domain experts who are often not well-trained in formal methods. E.g. [13,9]
list the problems that users encounter when working with OWL. While some
of the problems are purely semantic (e.g. caused by misunderstanding the open
world reasoning and the unique name assumption) and would be encountered in
any syntax, many problems are rooted in the nature of current OWL syntaxes.
Furthermore, many knowledge bases require a rule component, often expressed
in SWRL. The proposed SWRL syntax, however, is completely different from the
1
http://protege.stanford.edu/
OWL syntaxes (mainly because it explicitly uses variables) even though there
is an overlap of the semantics of OWL and SWRL. Query languages for OWL
ontologies introduce yet another set of syntaxes.
The syntactic complexity can be hidden to some extent by front-end tools
such as Protégé which use various user interface widgets to support viewing and
editing knowledge bases. For example, the sub class hierarchy of named classes
can be presented as a tree. Still, the relative richness of OWL and related lan-
guages means that for more complex expressions (negation, property restrictions,
etc.), the user interface has to fall back to one of the standard syntaxes.
An alternative and less explored approach is to base ontology editing on
the use of controlled natural language (CNL) [15]. Several studies have shown
that controlled English can offer an improved usability over existing approaches
for domain experts working with OWL statements [10,4,5]. However, ontology
editors that offer CNL-based interaction as their main component are still in
their infancy and their possible architecture has not been agreed upon.
This paper describes ACE View, an ontology and rule editor that offers the
creation, viewing, editing and querying of the logical content of OWL 2 ontolo-
gies and SWRL rulesets in ACE. ACE View offers one unified syntax for OWL
axioms, SWRL rules and DL queries — axioms and rules are expressed as En-
glish declarative sentences and queries as English interrogative sentences. ACE
View is implemented as an extension to the popular ontology editor Protégé.
This greatly simplifies the implementation of our approach as we can leverage
the integrated OWL API [6], reasoners, rule and query support that Protégé pro-
vides and just concentrate on providing the controlled English front-end to these
features. Also, we can easily fall back to the Protégé solutions for e.g. annotation
editing, etc. that we do not intend to express in ACE. Being based on Protégé
also simplifies the evaluation of our approach, e.g. one sensible way to evaluate
ACE View is to let users complete an ontology engineering task and observe for
how much of it they want to fall back to the standard Protégé approach.
This paper is structured in the following way. In section 2 we give a short
overview of ACE and the mappings between ACE and OWL/SWRL, in section 3
we describe the main features of the ACE View editor, in section 4 we review the
related work, and finally, in section 5 we draw conclusions and describe future
work.
2 ACE ⇔ OWL/SWRL
ACE is a subset of English, such that each sentence in the chosen subset is
interpreted unambiguously, relating the sentence to a unique logical form. The
intention behind the design of ACE is to offer expressivity required in knowl-
edge engineering tasks, but also remain a natural subset of English. The design
minimizes what the users need to learn (assuming knowledge of English) to cor-
rectly compose ACE sentences and understand their meaning, e.g. to predict the
paraphrases or the logical entailments of the sentences.
In order to make ACE interoperable with some of the existing Semantic Web
languages, mappings have been developed to relate ACE to OWL, SWRL, and
DL-Query (see a detailed description in [8]). For example, the mapping of ACE
to OWL/SWRL translates the ACE text
Every employee that does not own a car owns a bike.
Every man that owns a car likes the car.
Which car does John own?
into a combination of OWL axiom, SWRL rule and DL-Query (an OWL class
expression).
employee u ¬ (∃ own car) v ∃ own bike
man(?x) ∧ own(?x, ?y) ∧ car(?y) → like(?x, ?y)
car u ∃ own− {John}
The OWL→ACE mapping, on the other hand, allows us to verbalize existing
OWL ontologies as ACE texts. This mapping is not just the reverse of the
ACE→OWL mapping as it also covers OWL axiom and expression types that
the ACE→OWL mapping does not generate. For example, the OWL axiom
PropertyDomain(ObjectProperty(write) Class(author))
is verbalized as “Everything that writes something is an author.”.
The mappings between ACE and OWL/SWRL provide an alternative syntax
for OWL and SWRL. This syntax is readable as standard English and provides
linguistically motivated syntactic sugar. It also makes the difference between
OWL, SWRL and DL-Query invisible. This syntax is mainly intended for struc-
turally and semantically complex OWL/SWRL knowledge bases for which visual
methods and traditional syntaxes fail to provide a user-friendly front-end.
3 ACE View
3.1 Introduction
The ACE View editor provides an alternative view to the ontology and its entities
— a natural language rendering of the complete logical content of the ontology
where for the natural language we use ACE. In this rendering, ACE sentences
correspond to OWL axioms, ACE words to OWL entities, and most metrics are
linguistic, e.g. number of sentences and content words in the ACE text.
The ACE View editor lets the user manage an ACE text. The ACE text is
a set of ACE snippets where each snippet is a sequence of one or more ACE
sentences. The sentences in the snippet can be anaphorically linked. Usually,
however, a snippet contains just a single sentence. When a snippet is added
to the text, it is automatically parsed and converted into OWL/SWRL. If the
translation fails then the snippet is still accepted, it simply does not have any
logical axioms attached and thus cannot participate in reasoning. In case the
translation succeeds, the snippet is mapped to one or more OWL axioms and
SWRL rules which are also merged into the Protégé managed ontology. In case
a snippet is deleted, its corresponding axioms (if present) are removed from the
underlying ontology.
Alternatively, the ACE View user can switch to one of the standard Protégé
views to perform an ontology editing task. In case an OWL axiom is added in
the standard view, then it is automatically verbalized into at most one snippet
and merged into the ACE text.2 If the verbalization fails (e.g. the verbalizer
does not support the FunctionalProperty-axiom with data properties) then an
error message is stored and the axiom is preserved in the ACE text in Manch-
ester OWL Syntax. In case an axiom is deleted, then its corresponding snippet is
deleted as well. Note that the deletion operations can sometimes be quite com-
plex because the deleted axiom might have been generated by several different
snippets. Furthermore, each of those snippets might have generated also other
axioms.
The ACE text (and thus the ontology) can be viewed and edited at several
levels (word, snippet, vocabulary, text). Word level provides an access to OWL
entities in the ontology and allows one to specify how the entities should ap-
pear in ACE sentences, i.e. what are the surface forms (e.g. singular and plural
nouns and verbs) of the words that correspond to the entities. Entities can be
further annotated using the standard Protégé views. Snippets can be catego-
rized as asserted declarative snippets, asserted interrogative snippets (i.e. ques-
tions) and entailed (declarative) snippets. Asserted snippets are editable and
provide access to their details (parsing results such as error messages or syn-
tax trees/syntax aware layout, corresponding axioms/rules, ACE paraphrase).
Questions provide additionally answers. Entailed snippets are not editable but
can be explored to find out the reasons that cause the entailment. Vocabulary
is a set of ACE content words. It can be sorted alphabetically or by frequency
of usage. As content words correspond to OWL entities, standard Protégé views
offer even more presentation options, e.g. the “back-bone hierarchy” of sub class
and “part of” relations; separation of the vocabulary into classes, properties,
individuals. The vocabulary level provides an quick access to the word level,
each selected/searched word (entity) can be automatically shown in the word
level, or its corresponding snippets in the text level. An ACE text is a set of
ACE snippets. This set can be filtered, sorted, and searched. Reasoning can be
performed on the whole text to find out about its (in)consistency. A new text
can be generated by filling it with snippets that the asserted text entails.
3.2 Views
The ACE View editor comprises several views, most of which are editable. Some
views show what has been asserted, some show what is entailed. The following
screenshots show working with the “People and pets” ontology3 which we ver-
balized automatically in ACE and edited slightly to make it conform to the best
2
The default Protégé user interface currently only allows SWRL rules to be viewed,
but not edited and created.
3
http://protege.cim3.net/file/pub/ontologies/people.pets/people+pets.owl
Fig. 1. One possible layout of the ACE View editor. Several views are shown:
alphabetically ordered list of ACE content words (with frequency of usage in
parentheses); ACE Snippets table, sortable by the length of the snippet, number
of system messages, etc.; ACE Lexicon editor, highlighting the selected word
and showing its morphological forms; ACE Word Usage view, showing all the
snippets that contain the selected word.
practices of writing ACE (e.g. the construct ‘has part’ was replaced by ‘is a part
of’).
Vocabulary and wordform views (figure 1) In the “Lexicon view” and
“Words view”, the complete content word vocabulary of the ACE text is pre-
sented, sorted either alphabetically or by frequency of usage. Clicking on one
of the words, “selects” this word signaling the other views (e.g. “Word Usage
view”) to display information about this word. In English, a content word can
take several forms when used in a sentence. For example, the transitive verb ‘bor-
der’ (which maps to an object property ‘border’) can be used in three different
forms.
Every country that borders more than 2 countries that border a coun-
try that is bordered by a sea . . .
The “Lexicon view” allows the user to edit such surface forms and make
sure that they all correspond to the same OWL entity. When a new entity is
generated in the standard Protégé views, the surface forms of its corresponding
content word are automatically generated based on rules of English morphology.
The user can override these forms if needed.
Views to asserted knowledge (figure 1) The “Snippets view” organizes all the
asserted snippets in a table. With each snippet a set of its features are presented:
snippet length (in content words), the number of system error/warning messages,
creation time, etc. The table rows can be highlighted and filtered based on the
selected word, presenting only the snippets that contain the word. The “Snippet
Editor” (figure 2) shows the logical and linguistic properties of the selected
snippet and lets the user to edit the snippet. For sentences that fail to map to
OWL/SWRL, error messages are provided. Error messages point to the location
of the error and explain how to deal with the problem. Although the user is
expected to enter sentences that can be mapped to OWL/SWRL, inputting
sentences that are not compatible with the ACE→OWL/SWRL mapping or that
are not even in ACE is tolerated. Such sentences, however, do not contribute to
the text logically (e.g. they do not participate in reasoning). Such sentences can
be modified at any time to make them comply with OWL/SWRL.
A paraphrase is one way for the user to check if his/her interpretation of the
inserted text is accurate. ACE provides many forms of syntactic sugar, allow-
ing to paraphrase every-sentences as if-then sentences, verb phrase negation as
sentence negation, etc.
Fig. 2. ACE Snippet Editor. On the left: a successfully parsed sentence pretty-
printed; feedback to the user in the form of a paraphrase and corresponding
logical axioms; meta information in the form of OWL axiom annotations of the
corresponding axioms. On the right: a sentence with a syntax error and an error
message that points to the location of a missing determiner.
Views to entailed knowledge (figure 3) The “Q&A view” lists ACE ques-
tions and answers to them. These questions correspond to DL-Queries which
are essentially (possibly complex) class expressions. The answers to a DL-Query
are named individuals (members of the queried class) or named classes (named
super and sub classes of the queried class). In ACE terms, the answers are ACE
content words — proper names and common nouns. While the answers to DL-
Queries are representation-wise identical in the ACE view and in the standard
Protégé view, the construction of the query is potentially much simpler in the
ACE view, as one has to construct a natural language question.
The “Entailments view” provides a list of ACE sentences that follow logically
from the ACE text, i.e. these sentences correspond to the entailed axioms of the
ontology. Such axioms are generated by the integrated reasoner on the event of
classification. These axioms have a very simple structure, i.e. they are class asser-
tions, property assertions and sub class axioms where the involved individuals,
properties, and classes are always named. Complex class expressions including
e.g. property restrictions are not part of the entailed axioms as provided by the
reasoners currently integrated into Protégé. As the entailments are structurally
simple, their natural language verbalization does not bring significant usability
improvement over a traditional OWL syntax. Nevertheless, the presentation of
all entailments as a single list of natural language sentences can provide a good
and easily readable overview.
Protégé also supports entailment explanations. Such an explanation is a se-
quence of previously asserted axioms that motivate the entailment. The axioms
in this sequence can be of any complexity and thus their natural language pre-
sentation can bring significant improvement in understanding the reason behind
the entailment.
3.3 Benefits of using ACE View
As already discussed above, using ACE View brings several benefits to a domain
expert building an ontology. In this section we list three more concrete beneficial
features of ACE View.
Naming conventions and consistency By using English sentences as the
main component of the user interface, ACE View puts a natural restriction on
the orthography of OWL entity names and makes sure that the names are used
consistently. For example, it would be quite hard for the user to mix the use
of both singular nouns and plural nouns for class names. It would also be quite
unnatural to use camel case in anything else than proper names (e.g. “easyJet”).
A similar naming style is also promoted by the biomedical ontologies community
[14].
Structural complexity of class expressions ACE View places a natural
restriction on the complexity of OWL class expressions which by OWL syntax
can be arbitrarily long lists and embed into each other to arbitrary depth. As
Fig. 3. The “Q&A view” lists all the entered questions with their automati-
cally generated answers. The “Entailments view” lists the entailments that the
ACE text makes. Clicking on an entailed snippet reveals the explanation — the
asserted snippets that cause the entailment. Both the answers and the entail-
ments/explanations are updated whenever the reasoner is run.
OWL classes are expressed by ACE noun phrases, users are more likely to create
structurally simpler class expressions which in turn remain more human-readable
in whatever syntax the are later displayed.
Fall-back to existing syntaxes/methods OWL 2 includes powerful short-
hand axioms like DisjointUnion, motivated by common OWL usage patterns.
ACE does not provide such short-hands and the corresponding ACE snippet
will therefore unravel the complex construction via simpler ones. For example,
DisjointUnion(Class(person) Class(male) Class(female))
would be verbalized as
Every person is a male or is a female. Everything that is a male or that
is a female is a person. No male is a female.
While this is a valid approach that explains the notion of a covering union
of pair-wise disjoint classes to a novice OWL user, more experienced OWL users
may prefer a more concise representation that would give a better overview of
what is being said. For that, ACE View allows the user to fall-back to a standard
Protégé solution, e.g. a visual expression (DisjointUnion can be expressed as a
pie chart) or Manchester OWL Syntax.
Also, many important statements in the ontology, e.g. about the scope and
the purpose of the ontology, are non-logical and thus not handled by ACE View
via ACE sentences. As ACE View is implemented as a Protégé plug-in, it is
very easy for the user to switch to standard Protégé annotation views for such
meta-annotation tasks. Protégé also provides important editing support, such as
refactoring (e.g. renaming entities), storage (ACE snippets are stored as axiom
annotations and morphological surface forms as entity annotations), search, etc.
ACE View will immediately profit from future developments in Protégé (e.g.
collaborative editing).
3.4 Implementation and availability
The ACE parser has been implemented in SWI-Prolog and released under the
LGPL open source license. The distribution also includes the translator from
ACE to OWL/SWRL4 . The ACE parser is also available as a REST-webservice5 .
The verbalization of OWL ontologies has been implemented in SWI-Prolog
and is publicly available as a REST-webservice6 that accepts ontologies in OWL
2 XML serialization as input and produces the an ACE text as output.
ACE View is implemented as a plug-in for Protégé 4 and relies heavily on
the OWL API [6] that provides a connection to reasoners, DL-Query support,
entailment explanation support, storage of OWL axioms and SWRL rules in the
same knowledge base, etc. The main task of the ACE View plug-in, translating to
and from OWL/SWRL, is performed by the two translator webservices. For the
morphological generation of entity surface forms, we use the Lexicon Generation
API7 by Albert Gatt. ACE View plug-in is available in binary form8 . We are
currently working towards releasing it under an open source license.
4 Related work
AceWiki9 is a “semantic wiki” [10,11] that uses ACE as its underlying formal
language. Similarly to ACE View, it uses the ACE→OWL/SWRL mapping to
enable reasoning over the wiki content. Semantic feedback is provided via ques-
tion answering and consistency checking. An interesting feature of AceWiki is
that editing of the wiki content is supported by a predictive editor, which in
addition to auto-completing content words is also “syntactically aware” of the
following context and can thus guide novice ACE users in forming syntactically
correct ACE sentences.
The ROO tool [1] uses the Rabbit language [5] as its underlying controlled
English. It is similar to ACE View as it has also been implemented as an exten-
sion to Protégé 4, although all of the default Protégé views have been stripped
from the user interface. ROO allows entering Rabbit sentences which are then
4
http://attempto.ifi.uzh.ch/site/downloads/
5
http://attempto.ifi.uzh.ch/site/docs/ape webservice.html
6
http://attempto.ifi.uzh.ch/site/docs/owl to ace.html
7
http://www.csd.abdn.ac.uk/∼agatt/home/links.html
8
http://attempto.ifi.uzh.ch/aceview/
9
http://attempto.ifi.uzh.ch/acewiki/
translated into OWL automatically. The other direction of viewing existing OWL
axioms as Rabbit sentences is not possible. Also, ROO does not provide any se-
mantic feedback to the users. ROO was compared to ACE View in an evaluation
involving 16 students who were beginners in conceptual modeling and OWL [2].
ROO was found to outperform ACE View in certain usability aspects. It must
be noted though that the evaluation focused mainly on the syntactic aspects of
using a CNL-based interface (e.g. how useful are the syntax error messages that
the tool provides), and that some of these aspects have been improved in ACE
View since the evaluation.
The controlled English CLoNE is being used in a GATE10 -based ontology
editor [4]. CLoNE, however, is quite a simple language (defined by 11 sentence
patterns) and does not therefore provide the same syntactic and semantic ex-
pressivity as offered by ACE View.
A longer overview of work related to (C)NL-based ontology editors and query
interfaces is provided by [16].
5 Conclusions and future work
ACE View introduces a novel paradigm to OWL/SWRL engineering. We as-
sume that ontologies and rulesets are usually first expressed (in the minds of
domain experts) in natural language, and thus working in ACE involves fewer
conceptual problems. On the other hand, the combination of natural language
based ontology editing and the standard form/formula-based editing offers more
alternatives for the user and can result in an interesting synergy especially in the
case of novice ontology engineers and domain experts working with semantically
expressive and syntactically complex knowledge bases.
Our future work will focus on providing even more semantic feedback to the
user, e.g. we would like to offer redundancy checking of the ontology, tracking
changes in the entailments and answers (this can be seen as unit testing or re-
gression testing known from software engineering), more choice for paraphrasing
the asserted snippets, etc. To support the user syntactically, a predictive editor
(e.g. the one used in AceWiki) could be integrated into ACE View.
Acknowledgment
This research has been funded by the European Commission and by the Swiss
State Secretariat for Education and Research within the 6th Framework Pro-
gramme project REWERSE number 506779 (cf. http://rewerse.net). The
author is currently supported by the Swiss National Science Foundation (grant
100014-118396/1).
The author would like to thank the three anonymous reviewers of OWLED’08
for their valuable feedback. Also, the author would like to thank Norbert E.
Fuchs, Tobias Kuhn, and Fabio Rinaldi for useful comments on the draft of this
paper.
10
http://gate.ac.uk/
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