=Paper=
{{Paper
|id=Vol-2364/2_paper
|storemode=property
|title=Creating vocabulary exercises through NLP
|pdfUrl=https://ceur-ws.org/Vol-2364/2_paper.pdf
|volume=Vol-2364
|authors=Manex Agirrezabal,Begoña Altuna,Lara Gil-Vallejo,Josu Goikoetxea,Itziar Gonzalez-Dios
|dblpUrl=https://dblp.org/rec/conf/dhn/AgirrezabalAGGG19
}}
==Creating vocabulary exercises through NLP==
https://ceur-ws.org/Vol-2364/2_paper.pdf
Creating vocabulary exercises through NLP
Manex Agirrezabal1[0000−0001−5909−2745],
2[0000−0002−4027−2014]
Begoña Altuna , Lara Gil-Vallejo3[0000−0002−9393−5651], Josu
2[0000−0001−5568−4014]
Goikoetxea , and Itziar Gonzalez-Dios2[0000−0003−1048−5403]
1
University of Copenhagen,
Department of Nordic Studies and Linguistics. Manex.Aguirrezabal@hum.ku.dk
2
Ixa Group, University of the Basque Country, Manuel Lardizabal
1, 20018 Donostia. {begona.altuna,josu.goikoetxea,itziar.gonzalezd}@ehu.eus
3
Universitat Oberta de Catalunya
lgilva@uoc.edu
Abstract. The use of technologies in Humanities opens new research op-
portunities as it allows the access to vast amounts of data such as textual
corpora. As, in the Digital Humanities domain, a considerable amount of
the research is done on digitised corpora, Natural Language Processing tools
can be of much help in their exploitation for they help extracting linguistic
information. We present a series of experiments in which we propose text
transformations to generate vocabulary learning exercises based on Natural
Language Processing. We describe the corpus, databases and tools we have
employed in our approach and we offer an overview of a multilingual language
processing pipeline. Then we present the experiments and their output. We
finally discuss the strengths and shortcomings of our approach.
Keywords: Natural Language Processing·Text transformation·Vocabulary
learning.
1 Introduction
The increasing use of Information and Communication Technologies has opened new
research possibilities and the growing amount of digital data and the development
of processing tools have changed the paradigms of many research fields. In the case
of Humanities, the so-called Digital Humanities (DH) aim at exploiting the vast
amounts of digitised corpora with the help of the Natural Language Processing
(NLP) tools among others. In fact, DH and NLP can be considered closely related
fields as Humanities are often based on textual data and language knowledge. NLP
comprehends a wide range of research interests and approaches that can be useful
in DH in the NLP’s aim of providing computers with human language knowledge.
Since the 60’s, NLP’s challenges have mainly been Speech Recognition, Natural
Language Understanding and Natural Language Generation and some of its main
purposes are spell checking, parsing, machine translation, information retrieval, ques-
tion answering. However, it also addresses some more advanced applications such as
assistance for human creativity and uses in education. Our work can be placed in
those advanced uses of NLP.
19 Agirrezabal et al.
In what concerns enhancing human creativity, the so-called computational cre-
ativity is a notably prominent field. Computational creativity aims at modelling,
simulating or replicating human creativity using computational models and, hence,
it can help to enrich human productiveness providing suggestions, as it is the case
of automatic poetry generation [16]. For example, Agirrezabal et al. [4] describe word
substitutions based on Part-of-Speech and meaning in order to create new poems
form existing ones preserving the coherence of the texts. Although that approach is
focused on poetry, the substitutions presented can be of much help in other tasks
dealing with vocabulary substitution.
Regarding the educational usage, one should consider that NLP has also largely
been used for exercise generation and for student assessment [5, 18]. In fact, automa-
tising exercise generation can be considered a hybridisation of both computational
creativity and educational use of NLP, as automatic exercise generation may offer
suitable options where human creativity struggles —it can save time to teachers and
textbooks designers or can be useful for self-learning.
Our experimentation is centred, thus, on reusing NLP tools for computational
creativity and educational purposes. In this paper we present a proposal of a set of
NLP resources and tools for text adaptations to be used in the areas of language
teaching and inclusion. More precisely, we focus on text transformations that can
be used in the language acquisition field e.g. creating vocabulary exercises.
We strongly aim at the reusability of our proposals and, hence, all the resources
and tools used in our work (corpora, databases and tools) are freely available. More-
over, we provide the code we have created4 under the Creative Commons licence,
international version (4.0) and NonCommercial attribution (CC 4.0 BY-NC).
This paper is structured as follows: in Sections 2 and 3 we describe the resources
and tools we use in our experiments; in Section 4 we present the texts transformations
we propose and in Section 5 we discuss certain issues arisen from our experiments.
Finally, we conclude and outline the future work in Section 6.
2 Linguistic Resources: Corpus and Databases
Our work is prominently based on text and, precisely, our two main resources are a
narrative corpus collected by us (Section 2.1) and the WordNet [14] database (Section
2.2). In addition, we have also taken advantage of the ImageNet [13] database, in
which images are related to concepts and mapped according to those (Section 2.3).
2.1 Fairy tales Corpus
In our experiment, we have opted for children’s literature to build our corpus; more
precisely fairy tales we have extracted from Project Gutenberg5 and Wikipedia.
Project Gutenberg offers over 57,000 freely available e-books in 67 languages. In the
case of Wikipedia, each tale is indexed as an individual Wikipedia entry which offers
4
https://github.com/dss2016eu/codefest/tree/master/nlp lac
5
http://www.gutenberg.org
Creating vocabulary exercises through NLP 20
the background of the story as well a version of the tale. The reasons for opting for
well-known fairy tales are the following:
– Folktales have been widely employed in education [19].
– They are optimal for language learning for they are commonly told in easy
language and are widely known [19].
– There is a wide range of fairy tales freely available.
– Versions of those tales can be easily found for a myriad of languages and mul-
tilingual approaches can be easily implemented.
– Text from Project Gutenberg and Wikipedia can be easily obtained as plain
unformatted texts, which simplifies the textual preprocessing stage.
In order to give an idea of the size of the corpus we have compiled, we have listed
the token amounts for each tale and language in Table 1. As can be noticed, we
have tried to gather texts of similar length in order to achieve comparable results
in different languages.
Language Little Red Hansel and Total
Riding Hood Gretel
English 1384 2870 4254
Spanish 564 2637 3201
Catalan 563 647 1210
Galician 497 93 590
French 1432 2680 4112
German 1257 2663 3920
Italian 1199 1946 3145
Portuguese 662 2610 3272
Dutch 1311 2726 4037
Basque 274 174 448
Table 1. Tokens per document and language
In this paper, we will illustrate our work through selected passages from the Little
Red Riding Hood tale by the Grimm brothers.
2.2 WordNet
WordNet6 is a large lexical database of English [14] where nouns, verbs, adjectives
and adverbs are grouped into sets of cognitive synonyms (synsets). For example,
the words car, auto, automobile, machine, and motorcar are grouped in a synset
which denotes the concept a motor vehicle with four wheels; usually propelled by
an internal combustion engine. Moreover, the synsets are related among them. The
most important semantic relations are hypernymy-hyponymy, meronymy, troponymy
(for verbs) and antonymy. In Figure 1 we present the synset car, auto, automobile,
machine, motorcar and its related words.
6
http://wordnetweb.princeton.edu/perl/webwn
21 Agirrezabal et al.
Fig. 1. Main semantic relations in WordNet
However, it should be taken into account that a word may have more than one
meaning. When entering a query for a certain word we find all the word’s senses listed.
E.g. if we look for the noun hood, we may find these three senses among others: i) a
headdress that protects the head and face ii) protective covering consisting of a metal
part that covers the engine and iii) the folding roof of a carriage. Hence, it is plausible
for a word to appear in more than one synset depending on which sense is considered.
Following the English WordNet philosophy, WordNets for many languages have
been developed. For example, during the EuroWordNet (EWN) project7 WordNets of
several European languages (English, Dutch, Italian, Spanish, German, French, Czech
and Estonian) were created. Nevertheless, these WordNets for different languages
are not isolated databases. The Inter-Lingual-Index (ILI) was created to provide an
efficient mapping across the autonomous WordNets. Via this index, languages are
interconnected so that it is possible to go from the words in one language to the
equivalent words in any other language listed.
The list of available WordNets for different languages has been increasing since
then. For example, the Open Multilingual WordNet [9] (OMW)8 —a product of
the Global WordNet Association9— provides access to open WordNets in over 150
languages, all linked to the English WordNet.
In this work, we have chosen WordNet for our experimentation because i) it is
a well-known multilingual and ii) a freely available resource and iii) it has been used
in many NLP applications. Specifically, we have used the OMW that is included
in NLTK [8], which is a suite of Python libraries and software for symbolic and
statistical NLP, and we have used it to substitute words (the lemmas, exactly) with
semantically related concepts or with its equivalents in other languages.
2.3 ImageNet
ImageNet10 [13] is a large-scale image database arranged according to the hierarchy in
WordNet. It contains images for nouns and each node of the hierarchy is represented
by thousands of images. That is to say, nouns in the English WordNet get images
7
http://projects.illc.uva.nl/EuroWordNet/
8
http://compling.hss.ntu.edu.sg/omw/
9
http://globalwordnet.org/
10
http://www.image-net.org
Creating vocabulary exercises through NLP 22
that represent them. More precisely, the ImageNet project aims at offering 500-1000
images for each synset. As WordNet, ImageNet is freely available and ready to use.
Although ImageNet was initially developed for visual information processing and
tasks such as non-parametric object recognition, tree-based image classification and
automatic object localization, it has been proven very useful in our experiment. As a
matter of fact, in this work we have employed ImageNet for the substitution of nouns
in texts for images. By means of that, we have been able to create texts with images
similar to texts with pictograms. We further describe that approach in Section 4.2.
3 Preprocessing with NLP tools
Textual processing has been done by some existing off-the-shelf NLP tools. The re-
quired processing for most of the languages (Basque, Dutch, English, French, German,
Italian and Spanish) has been done through Ixa-pipes11 [1], whereas the processing for
the rest of languages (Galician, Catalan and Portuguese) has been conducted through
FreeLing12 [11]. Although we have employed two different processing pipelines, the
steps are comparable. As a consequence, we will describe the processing modules
relevant to this work based on Ixa-pipes’ performance.
Ixa-pipes is a modular chain of NLP tools (or pipes) which provide easy access to
NLP technology for several languages. Modular means that the different processing
modules (for specific linguistic analysis tasks) can be chosen according to the needs
of each experiment and that new modules can be added to address new needs. We
present the processes we have carried out below:
– Tokenisation: it is the process of splitting sequences of characters into minimal
meaningful units. In the tokenisation process, texts are divided into words,
numbers, acronyms or punctuation marks. As can be seen in Figure 2, the
sentence (sent=“3”) has been split into tokens (wf ) and each token has been
assigned an identifier (id). Tokenisation parameters are defined for each language
so as to take into account the special characters and singular orthography and
punctuation rules each language may have. It is also to be pointed out that
punctuation and hyphenation exceptions have been taken into account as can be
seen for the Red-Cap token, which has been considered a single unit despite the
fact there is a hyphen involved.
– Lemmatisation: it consists in removing word inflection to return the dictionary
form or lemma of a word. For example, from the verb form is we obtain the
lemma be after lemmatisation is done. In Ixa-pipes lemmatisation is performed by
lexical look-up methods in which each word in text is checked in a dictionary. The
lemma is the basis in our experimentation, since we create the transformations
based on it. Consequently, an unknown or incorrect lemma can lead to errors in
the following processes.
– Part-of-Speech tagging: it consists in assigning a grammatical category to
each of the tokens. In Ixa-pipes this is a two step procedure: first, all the possible
11
http://ixa2.si.ehu.es/ixa-pipes
12
http://nlp.lsi.upc.edu/freeling/node/1
23 Agirrezabal et al.
Fig. 2. Tokenisation example for Little Red Riding Hood
analysis are assigned to each token and then, the most suitable one is selected. In
this process both linguistic knowledge (rules) and statistical methods are combined.
In Figure 3, we present the complete annotation of a segment of the Little Red
Riding Hood tale in English. One may notice that each token is presented in
blue, lemmas are expressed by the lemma attribute and Part-of-Speech (PoS)
information is given in the pos attribute.
– Word Sense Disambiguation (WSD)[2]: it is a NLP task that aims to
identify the sense of a word in a sentence when that word has more than one
sense. For example, given the sentence We took off the hood, the goal is to assess
whether the word hood refers to the headdress or to the car cover. In order
to perform WSD in this work, we have used the state-of-the-art tool UKB [3],
which is also integrated in Ixa-pipes, and works with English, Basque, Bulgarian,
Portuguese and Spanish. As an output, UKB offers all the possible WordNet
synsets (reference) with a confidence value (confidence attribute) as we show in
Figure 4 for the word hood.
This is the preprocessing needed in order to carry out the text transformations
presented in Section 4.
4 Text Adaptations
Once textual processing has been done, we have profited from the extracted linguistic
information to alter texts and generate reading and vocabulary activities automat-
ically. In the following subsections we describe the kind of exercises we have created
for helping to acquire language.
Creating vocabulary exercises through NLP 24
Fig. 3. Lemmatisation and PoS tagging example for Little Red Riding Hood
Fig. 4. WSD analysis for the lemma hood
4.1 Word clouds: working with global comprehension and vocabulary
Our first experiment focuses on the pre-reading stage, in which we aim at enhancing
text comprehension by going through the plot of the story or the characters. More
precisely, we have created word clouds which visually highlight the main ideas of the
texts. Word clouds are visual representations of the words in a text and are typically
used to depict the keywords. Commonly, most frequent words are displayed in bigger
sizes, and thus give a straightforward insight on the topic of the text.
In fact, frequency of words shows a lot of potential in order to sketch the
information in text, as high-frequency items cover a large proportion of words in text.
25 Agirrezabal et al.
Hence, they have been worthy of attention by both language teachers and learners
[23]. Furthermore, word clouds are a widely considered teaching resource [6, 12, 21, 29].
Taking all that into account, word clouds are convenient for a first introduction of
texts as they offer a global view of the plot and can be a good tool to deal with the
most relevant or specific vocabulary from a visual and playful approach. Additionally,
word clouds can also be useful to compare two different texts, two authors on the
same topic, namely.
In what regards the word cloud generation system, we have developed a prototype
of a word cloud generator by combining the packages Matplotlib and Numpy in
the Python programming language. All words from the original text are shown
in the word cloud, except digits, punctuation marks and the so-called stop words
(prepositions, determiners, conjunctions, etc.) that do not convey relevant information
on the topic, in order to focus on meaningful words. We generate word clouds with
the shape of an input image relevant to the tale we want to deal with so as to make
the word clouds more appealing to the language learner.
In Figure 5 we present the final word cloud we have obtained from the Little
Red Riding Hood tale. As can be seen, the word cloud acquires the shape of Little
Red Riding Hood and the most frequent words are grandmother, little, red-cap and
good. Dealing with those words is useful to start working with the vocabulary and
the concepts the readers will find in the text.
Fig. 5. Word cloud for Little Red Riding Hood
Creating vocabulary exercises through NLP 26
4.2 Pictotales: working on vocabulary with images
Pictotales are the tales in which nouns have been replaced with images from ImageNet.
This is a single-language textual transformation approach in which some lemmas
have been replaced for pictures.
In order to create the Pictotales, lemmas of some of the concrete nouns have been
automatically looked up in ImageNet and an image corresponding to the lemma synset
has been randomly chosen. In order to combine text and images, the narratives have
been converted to HyperText Mark-up Language (HTML), which allows displaying
text and images in web browsers and other visual interfaces.
As can be seen in Figure, 6, we have created a Pictotale from the English ver-
sion of the Little Red Riding Hood tale. As one can see, concepts such as mother,
grandmother, and bottle have been replaced with some relevant images.
This kind of exercises can be useful to help to learn vocabulary, using the images
as contextualized flashcards or helping to evoke the target words in a first reading
of the tales. Furthermore, pictotales can also be employed for performing vocabulary
revision and memory exercises, naming the elements that appear in the images, all
within the context of the tale.
Fig. 6. Little Red Riding Hood with images
4.3 Story revolution: working on vocabulary with related words
In the Story Revolution experiment, we have aimed at vocabulary learning through
meaning-based substitutions. That is to say, we have substituted nouns and adjectives
in texts with their antonyms, hyponyms or hypernyms.
27 Agirrezabal et al.
Since we work with lemmatised texts, we have used the WordNet information
for those. As WordNet can be understood as a net of semantically-based relations in
which concepts are arranged from the most generic to the most specific and opposition
relations are also included, obtaining the antonyms, hyponyms or hypernyms of the
selected lemmas in text has been a straightforward process. Figure 7 displays a piece
of Little Red Riding Hood where some words have been replaced with their antonyms
(in red) e.g. few, large, ignore....
Fig. 7. Passage of the English Version of Little Red Riding Hood with Antonyms
Replacing words with their hypernyms may help on text understanding for second
language learners. In fact, removing the most obscure terms and offering more generic
alternatives may lead, at least, to getting the global sense of the texts. We present a
sentence of Little Red Riding Hood’s tale in (1). As it can be seen, some of the words
have been highlighted in different colours. In example (2), we present a sentence
formed by the hypernyms of those highlighted words. In this second sentence more
generic vocabulary is used and the sentence could presumably be better understood
by non-proficient speakers of English.
(1) When the wolf had appeased his appetite.
(2) When the canine had calmed his craving.
On the contrary, when substituting words with their hyponyms, we help enlarging
the available vocabulary and learning more specific words. In example (3) we present
the outcome of replacing the highlighted words in (1) with their hyponyms.
(3) When the coyote bear had appeased his stomach.
Word substitution is a common technique in lexical substitution and text simpli-
fication tasks, but generally word replacements are done with synonyms [10, 27, 15].
Nonetheless, hypernym and hyponym substitution can also be suitable for the task,
since replacing all the words referring to the same concept for a single term makes
us convey with one of the easy reading principles: “use the same term consistently
for a specific thought or object” [25].
4.4 Uncovering words: discovering unknown words
In this experiment we wanted to automatically create one of the typical vocabulary
learning exercises. Given a text in the target language, we have substituted some
Creating vocabulary exercises through NLP 28
words for their translations in the learner’s tongue. However, we have not opted for
the individual translations of the words, but we have used WordNet to assign the
term, in order to guarantee the terms share the same meaning.
For example, in Figure 8 we have taken as a basis the English version of the
Little Red Riding Hood tale and we have translated some words into Danish (in red,
bedstemor, rum...). Consequently, we have obtained a traditional “fill in the gaps”
exercise with mother tongue clues.
Fig. 8. Passage of Little Red Riding Hood in English with some words in Danish
Conversely, this technique can be applied in order to present the text in a language
that the learner speaks and translate several words into the target language. For
example, the text in Figure 8 could be used by English speakers learning Danish. In this
way, learners find a comfortable context in which they can focus on the vocabulary. One
possible exercise that can be done is encouraging the learners to make hypothesis about
what a word means given its surrounding context in order to uncover the story in text.
5 Discussion
In this paper we have presented four text transformations that not only can help
teachers to create exercises but also writers or editors to create new texts. In fact, our
approach may shorten the exercise generation time and can also help the creativity of
the professionals, namely when looking for a convenient translation. Nevertheless, we
have to underline that these texts need a revision before they are used. In particular,
we detail next three main types of shortcomings.
1. Sometimes the absence of a linguistic item prevents from addressing the trans-
formation query to generate a variant in the text. An illustration of this can be
seen in examples (1) and (3) in Section 4.3 where the same form (had appeased)
is offered for both the target term and its hyponym (autohyponymy). According
to the resources used, there is not a more specific way of referring to the event,
and probably it will also be difficult for an expert to come up with one.
2. Additionally, as pictures from ImageNet are chosen randomly among all the
images linked to a certain synset, the picture selected might not be the most
suitable according to the context. For example, in Figure 6 the word bottle is
represented by an empty plastic bottle, but could be better represented by a full
29 Agirrezabal et al.
glass bottle of wine. We reckon an optimal approach to pictotale generation would
be a system that takes into account the whole narrative context for image selection.
3. Some other issues, instead, arise from processing errors. In some cases, the incor-
rect Part-of-Speech tag assignation may lead to the substitution of a wrong word.
In other cases, incorrect word sense disambiguation may be a source of errors.
For example, in (4) we present a passage of the Little Red Hood in Spanish where
some words have been translated into Danish (as when uncovering words, Section
4.4). As depicted in the example, we have found that the Spanish word chica
(girl) has been substituted with the Danish word dreng (boy). This error seems
due to the fact that the Spanish lemmatiser gives the masculine form chico (boy)
as lemma. UKB disambiguates the masculine lemma as boy and our script relies
on UKB’s disambiguation to look up the Danish word.
(4) (...) nada que no le hubiera give a la dreng (...)
(...) nothing that not her had.SUBJ give to the.FEM boy (...)
‘(...) nothing that hadn’t been given to the(fem) boy.’
Moreover, in case of the verbs, as our tools substitutes lemmas, it is necessary
to fix the conjugation. An example of this is the Danish verb give (give) in (4),
which should be corrected to the participle givet (given). In order to overcome
this problem, natural language generation techniques that take the syntax of
the target language into account should be used. Nonetheless, offering only the
lemma does not invalidate the proposed exercise as guessing or generating the
right verbal tense from the context is also a possible exercise.
Despite the shortcomings, this method offers great capacity for text adaptations.
In this work we have applied our modifications to all the words, but that can be
easily customised. Possible customisations are substituting less frequent words, longer
words, complex words and keywords/keyphrases among others.
For example, regarding frequent and infrequent words, word frequency lists such
as Ogden’s Basic English word list or the ones that take word distributions into
account [7] can be employed in order to set the threshold of interest (very frequent,
frequent, normal, infrequent, not frequent).
In what concerns word length, the Plain Language guidelines the use short words
is recommended (a summary of guidelines can be found in [22]). Shorter words can
be easier to learn; however, the influence of word length does not seem to be the only
factor for our memory [17].
These text adaptation processes may also be enhanced with other current NLP
techniques, such as complex word identification [24]. In this preprocessing step for
lexical simplification, complex words and expressions are identified in order to replace
them later with simpler equivalent alternatives [26]. Our approach may be integrated
in this step so as to replace complex words and expressions with simpler equivalent
alternatives.
Besides, keyphrase detection, which deals with finding most important words in
texts, is another NLP task that has been approached in the context of information
extraction [20, 28] but that can easily help the creation of text adaptations for students
with special needs, by helping the readers to get the main ideas of the text.
Creating vocabulary exercises through NLP 30
Taking all that into account, we reckon that, although we have centred our ex-
perimentation in vocabulary learning exercise generation and that our system admits
some improvements, the tools and resources proposed in our experimentation can
be of much help in other DH research trends in which text transformations play a
significant role. Additionally, even if we have presented our transformation proposals
as isolated experiments, they can all be combined to address specific needs.
6 Conclusion and Future Works
We have conducted a series of experiments in which we have altered texts through
NLP methods in order to create resources for vocabulary learning. Exactly, text
adaptations have been i) creating word clouds, ii) texts with images, iii) texts with
different but semantically related works and iv) texts with translations. Our main
objective was to show a possible support to teachers/educators when creating vocabu-
lary learning or reading comprehension exercises by means of NLP applications. This
approach can also be seen as a computational creativity exercise as transformations
and suggestions have been automatically generated.
Our experiments offer just the first insights on what NLP can offer for textual
modification. Automatic methods are far from being perfect and still need human
supervision, but it is undeniable the fact that they ease the burden of coming up with
suitable ideas in certain contexts. Further, taking into account that our approach
is a NLP-based one, we consider that in the next steps conducting both qualitative
and quantitative evaluations in real scenarios is fundamental so that to measure the
actual performance of our implementations and to integrate our preliminary proposal
within the scientific framework.
In the case of educational purposes, a first step on improving our work should be
evaluating the text adaptations with target audiences in order to better understand
their needs; e.g. which words should be adapted or what new approaches they may
require. That is why we encourage the collaboration with other experts in the area
of humanities and, specially, within the domain of education.
Moreover, we think our proposal can be adapted to address more education needs.
We foresee the following applications: i) adapting the texts with pictograms, ii) going
further than words and substituting phrases iii) creating games removing words from
texts or giving their definitions in order to guess them, or iv) giving two words and
guessing their relation. Creating an online text adaptation application, where each
one can customise its text, is also one of our future goals.
Out of the education domain, we believe our approach may also have different uses.
For example, we think it might be useful for museums or other cultural institutions
for the creation of adapted or multimedia and interactive material. In the case of the
artistic creation of word clouds automatically extracting the most relevant words can
also be of great help. Finally, we want to reinforce the idea that using NLP methods
might impulse computational creativity for new kinds artistic expressions.
31 Agirrezabal et al.
7 Acknowledgements
We thank Codefest summer school for providing the infrastructure to begin this work.
We also thank Larraitz Uria for her contribution to the early steps of this work.
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