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  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Enhancing Transformer-Based Sentiment Analysis for the Rest-Mex 2025 Challenge: A Hybrid Strategy with Oversampling, Back-Translation, and Transformers</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Muhammad Imran</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Tayyab Rasheed</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Carlos Gómez-Rodríguez</string-name>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Department of Computer Science, COMSATS University Islamabad</institution>
          ,
          <country country="PK">Pakistan</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Universidade da Coruña, CITIC Departamento de Ciencias de la Computación y Tecnologías de la Información</institution>
          ,
          <addr-line>Campus de Elviña s/n, 15071, A Coruña</addr-line>
          ,
          <country country="ES">Spain</country>
        </aff>
      </contrib-group>
      <pub-date>
        <year>2025</year>
      </pub-date>
      <abstract>
        <p>This paper presents a sentiment analysis framework for the Rest-Mex 2025 challenge, focused on Spanishlanguage reviews of Mexican Magical Towns. The task involves predicting sentiment polarity (1-5), classifying attraction type (Hotel, Restaurant, Attraction), and identifying the correct town from a list of 60. To address class imbalance, we propose a hybrid augmentation approach combining oversampling and back-translation using both structurally similar and dissimilar languages. Two transformer-based models roberta-base-bne and twitter-xlm-roberta-base are fine-tuned on the augmented datasets. The hybrid strategy, particularly with the multilingual model, achieved the best results, demonstrating improved performance and generalization across all subtasks. Our system achieved 4th place in the overall sentiment analysis track of the Rest-Mex 2025 shared task competing against 35 participating teams which demonstrates the robustness of our approach in sentiment classification across multiple subtasks.</p>
      </abstract>
      <kwd-group>
        <kwd>eol&gt;Sentiment Analysis</kwd>
        <kwd>TripAdvisor reviews</kwd>
        <kwd>Transformers</kwd>
        <kwd>NLP</kwd>
        <kwd>Oversampling</kwd>
        <kwd>Back-translation</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>1. Introduction</title>
      <p>
        The sentiment analysis of the reviews provided by tourists plays a key role in the understanding of
the public opinion in the tourism sector [
        <xref ref-type="bibr" rid="ref1 ref2">1, 2</xref>
        ] especially where tourism is the primary business. The
analysis of Spanish-language reviews for Mexico’s Magical Towns (Pueblos Mágicos) introduces a
unique set of linguistic and contextual challenges[
        <xref ref-type="bibr" rid="ref3">3</xref>
        ]. The reviews often contain informal tone, regional
dialects, and culturally rooted expressions, which makes the task more complex than standard sentiment
classification [ 4]. Unlike past editions [5, 6, 7], the Rest-Mex 2025 challenge at IberLef [8, 9] outlines
a comprehensive framework that comprises three subtasks. The first is the prediction of sentiment
polarity on a scale of 1 to 5. The second involves classification of the type of destination (either hotel,
restaurant, or attraction). The third and final subtask is the identification of the corresponding Magical
Town from a predefined list of 60 towns.
      </p>
      <p>The field of sentiment analysis has seen widespread adoption of transformer-based architectures.
Models such as BERT, BETO, and XLM-R have shown strong results on various multilingual and
Spanishspecific datasets [ 10]. Prior to the advent of transformers, classical machine learning algorithms like
SVMs and logistic regression were commonly used, often relying on handcrafted linguistic features and
bag-of-words representations [11]. While these earlier approaches provided reasonable results, they
lacked the deep contextual understanding required for handling nuanced and complex reviews.</p>
      <p>The major limitation of existing models lies in the handling of class imbalance [12]. Sentiment
labels such as “very negative” or “very positive” are often underrepresented, as are reviews for certain
Magical Towns. The general-purpose nature of most pretrained models results in poor handling of
tourism-specific phrases. Furthermore, the joint modeling of sentiment, destination type, and town
identification remains a non-trivial challenge due to the multitask nature of the problem.</p>
      <p>To address the class imbalance and improve performance across all subtasks, we introduce three
targeted strategies. The first involves the use of oversampling [ 13] for underrepresented classes. The
second applies back-translation [14] using structurally similar and dissimilar languages to increase
the diversity and volume of minority-class data. The third combines both techniques in a hybrid
approach. For model training, we use the roberta-base-bne and twitter-xlm-roberta-base
checkpoints, fine-tuned independently on datasets prepared with each strategy. Our solution improves
the representation of low-frequency classes and enhances generalization across the three subtasks.</p>
    </sec>
    <sec id="sec-2">
      <title>2. Related Work</title>
      <p>Recent studies highlight the success of transformer architectures such as multilingual BERT,
XLMRoBERTa, and domain-specific models for sentiment and text classification. For instance, BETO has
shown improved performance on Spanish sentiment tasks, while multilingual models allow for
knowledge transfer across languages. Back-translation and data augmentation strategies have also gained
attention for mitigating data imbalance and enhancing robustness.</p>
      <sec id="sec-2-1">
        <title>2.1. Sentiment Analysis in the Tourism Domain</title>
        <p>Sentiment analysis has become a valuable tool in the tourism industry, ofering insights into traveler
experiences through user-generated content [15, 16, 17]. By analyzing reviews and feedback, it helps
stakeholders enhance services and understand public perception of destinations.</p>
        <p>The researchers in [18] contributed a novel LDA topic-based sentiment analysis approach to
analyze tourism reviews, combining topic modeling with lexicon-based sentiment analysis to extract
insights from TripAdvisor reviews about Marrakech. In their methodology they used data scraping,
pre-processing, LDA for the topic extraction, and sentiment analysis using the VADER and TextBlob,
they achieved the accuracies of 77.3% and 72.6%, respectively, which outperformed the JST model by
3–7.7%. The study’s limitations include reliance on rule-based sentiment analysis, which struggles with
irony and sarcasm, and a focus only on English reviews, limiting generalizability.</p>
        <p>The SALSA project addresses the computational bottleneck in syntax-aware sentiment analysis
by developing lightweight systems that combine fast syntactic parsing with explainable sentiment
classification, enabling SMEs to perform accurate large-scale sentiment analysis without
resourceintensive infrastructure [19]. The study introduces SEquence Labeling Syntactic Parser (SELSP), a
method that treats dependency parsing as a sequence labeling task to accelerate syntax-based sentiment
analysis while maintaining higher accuracy than conventional parsers (e.g., Stanza) and heuristic tools
(e.g., VADER), making it viable for real-world SA applications[20].</p>
        <p>In another research work, [21] contributed a novel deep learning-based approach to analyze sentiment
and topics in tourism-related tweets during the Covid-19 pandemic,they focused on the hospitality
and healthcare sectors. Their methodology combined VADER for sentiment analysis, LDA for topic
modeling, and an LSTM-RNN model for sentiment classification, achieving the test accuracies of 80.9%
for hospitality and 78.7% for healthcare. Their proposed model outperformed traditional machine
learning methods like random forest and SVM, which demonstrated higher eficiency in capturing
nuanced sentiments. However, the study had some limitations, such as reliance on Twitter data with API
restrictions, which limited the dataset size, potentially afecting the LSTM’s performance. Additionally,
the model struggled with sarcasm and irony, which are common challenges in sentiment analysis.
Despite these shortcomings, the research provided a valuable insights into the public sentiment during
the pandemic.</p>
        <p>By combining BERT-based sentiment analysis with social media data from Twitter and Instagram
to evaluate tourism in Granada, [22] contributed a novel approach. Their methodology involved the
training of a Spanish-Tourism-BERT model for the sentiment classification, they used hashtags to
identify the key tourist spots and their associated sentiments. The model achieved an accuracy of
75.7% for the Spanish texts, that outperformed the other classifiers, while Tweeteval was used for the
English texts which gave the 75.6% accuracy. However, the study had limitations, such as data collection
challenges due to Instagram’s API restrictions and the need for larger training datasets to improve
the sentiment analysis further. Despite these issues, the work provided valuable insights for tourism
managers to enhance the destination marketing and services.</p>
        <p>For Spanish tourism data, [23] introduced a novel multimodal sentiment analysis model, uniquely
integrating both text and image inputs along with a data quality framework. Their methodology includes
extracting opinions from social platforms, classifying sentiments separately in text and images using
SenticNet 5 (adapted to Spanish) and facial recognition, and then combining the results through
decisionlevel fusion. Their proposed model achieved 70% accuracy in text classification, 33% on the images, and
71% when both of the modalities were fused. However, the study acknowledges the limitations such
as low image quality that impact facial emotion recognition and challenges with informal text from
platforms like Twitter.</p>
        <p>The researchers in [24] contributed a new context-aware, target-oriented method for sentiment and
emotion analysis in tourism-related social media posts, focusing on specific aspects like attractions,
accommodation, and food. They proposed a dictionary-based framework that combines manual
annotation with lexicons, enabling more precise detection of emotions and sentiments linked to specific
tourism targets. Although exact accuracy percentages are not provided, the system achieved high
inter-annotator agreement (up to 92.3% for emotional words), showing strong reliability. However, the
study is limited by the small annotated dataset of only 475 tweets, which may not fully capture the
diversity of tourist opinions online.</p>
        <p>Finally, [25] provided a comprehensive review of how sentiment analysis (SA) has been applied in
tourism, ofering a novel integration of bibliometric, systematic, and thematic analyses. They used
VOSviewer to examine 111 papers from 2012 to 2021, clustering them into key research themes like
consumer behavior, big data, and recommendation systems. While the study doesn’t propose a specific
model or report accuracy figures, it efectively maps the research landscape and identifies SA as a
powerful tool in understanding tourist sentiment. However, the review they provided is limited by its
reliance on the Scopus database and they have done it for English-language sources only.</p>
      </sec>
      <sec id="sec-2-2">
        <title>2.2. Transformer-Based Approaches in Sentiment Analysis</title>
        <p>The introduction of transformer-based models has improved sentiment analysis by allowing a better
understanding of context in text. Unlike traditional machine learning techniques that depend on manual
features or basic word embeddings, transformer models like BERT, RoBERTa, and their multilingual
versions learn bidirectional representations that capture subtle sentiment signals in diferent languages.
These models perform well in various domains, including the tourism domain, where reviews often
include informal and emotional language. Their ability to adapt through fine-tuning makes them
suitable for the domain-specific tasks such as multilingual sentiment classification in tourism.</p>
        <p>The study by Sudhir et al. [26] presented a clear comparison of sentiment analysis methods, including
machine learning, deep learning, and transformer-based models. They tested several classifiers such as
Naive Bayes, SVM, LSTM, and BERT on the IMDB dataset. Among these, the BERT model achieved the
highest accuracy of 89.5%, while the BERT Large model with the UDA technique reached an accuracy
of 95.22%. The paper also pointed out that the rule-based and lexicon-based approaches face challenges
with complex text patterns such as sarcasm, and they often require frequent manual adjustments.</p>
        <p>In another study, Bashiri et al. [27] performed a detailed comparison of transformer models used
in sentiment analysis. They tested the BERT, RoBERTa, XLNet, ELECTRA, DistilBERT, ALBERT, T5,
and GPT models on a total of 22 datasets. The authors followed a step-by-step approach to evaluate
the accuracy and generalization ability of each model. Among all models, the T5 model gave the best
results on most datasets, showing high flexibility and the ability to generalize well. The XLNet model
was strong in detecting irony and product-related opinions, while the BERT and DistilBERT models
gave the lowest performance, even though they were more eficient. The study also highlighted that the
models still face problems with understanding sarcasm and idiomatic expressions, which are common
in user-generated reviews.</p>
        <p>Two novel transformer-based models for explainable sentiment analysis were introduced in [28],
focusing on generating extractive summaries to explain predictions. Their methodology involved a
hierarchical transformer model (ExHiT) and a simpler sentence-based model (SCC), both applied to the
IMDB dataset. The SCC model achieved the highest classification accuracy at 93.51%, while the ExHiT
model reached up to 92.77% depending on the merging strategy. The study also explored explainability
metrics, showing SCC performed best with 70.74% precision, but ExHiT showed improvement with
enhancements like sentence masking. A noted limitation is that ExHiT sometimes extracted less
interpretable summaries, especially without proper masking and embedding strategies.</p>
        <p>In their research Wang et al. introduced a novel multimodal sentiment analysis model called TEDT,
which uses a transformer-based encoder-decoder framework to fuse text, audio, and visual data [29]. The
main innovation lies in converting nonnatural language features into natural language representations
using a modality reinforcement cross-attention module and a dynamic filtering mechanism. Their
model achieved high accuracy (89.3%) on the CMU-MOSI dataset and 85.9% on the CMU-MOSEI dataset,
outperforming many existing methods. However, the TEDT model has high training time and struggles
with accurately interpreting sarcasm and subtle expressions, which limits its performance in complex
emotional scenarios.</p>
        <p>A wide range of BERT-based transformer models is reviewed by [30] specifically for text-based
emotion detection, providing insights into their performance, strengths, and weaknesses. They
analyzed models like BERT, RoBERTa, XLNet, and DistilBERT across several datasets such as SemEval,
EmotionLines, and ISEAR, highlighting how fine-tuned versions achieved strong results: for instance,
HRLCE with BERT achieved an F1 score of 0.7709. The novelty lies in the comprehensive comparison of
how diferent model architectures and training strategies afect emotion detection accuracy. However,
the paper noted that models often struggle with detecting mixed or subtle emotions and face challenges
like fixed input lengths and high computational costs.</p>
        <p>A novel BERT-based CBRNN model for sentiment analysis on social media data, aiming to handle
challenges like noisy text and contextual information loss, is proposed by [31]. The model combines
zero-shot classification for data labeling, a pre-trained BERT for semantic embedding, dilated CNN for
local and global feature extraction, and Bi-LSTM for capturing sequence dependencies. It achieved high
accuracy, with 97% on the US-airline dataset and 93% on the IMDB dataset, outperforming other models
in precision, recall, and AUC scores. However, the study noted the complexity of the hybrid model and
the increased training cost as limitations.</p>
        <p>In [32] AlBadani et al. introduced a novel Sentiment Transformer Graph Convolutional Network
(STGCN) that models text data as a heterogeneous graph and learns sentiment-related node representations
using transformer-based mechanisms. Their method combines BERT-based embeddings, graph structure
with TF-IDF and PMI-based edges, and Laplacian eigenvector-based positional encoding. The ST-GCN
model achieved high accuracy 95.43% on SST-B and 94.94% on IMDB outperforming several
state-ofthe-art models. However, the paper notes that the model’s performance is slightly sensitive to the
removal of low-frequency words and may require careful tuning of learning rates and epochs to avoid
overfitting.</p>
        <p>Overall, the existing body of work highlights the efectiveness of transformer-based models and
data augmentation techniques in addressing sentiment analysis challenges, particularly in multilingual
and domain-specific contexts. However, limited attention has been given to simultaneously handling
class imbalance, multilingual variation, and multitask objectives within the tourism domain. This gap
reinforces the need for more comprehensive and targeted approaches, such as the one proposed in this
study, to improve generalization and performance across all subtasks in the Rest-Mex 2025 challenge.</p>
      </sec>
    </sec>
    <sec id="sec-3">
      <title>3. Methodology</title>
      <p>The methodology in this study consists of three sections; exploratory data analysis to under the
distribution of the data instances, mitigating strategies to overcome the class imbalance issue and model
training and evaluation.</p>
      <sec id="sec-3-1">
        <title>3.1. Exploratory Data Analysis</title>
        <p>It’s important to understand the structure and distribution of the individual instances in dataset as
suggested in the study [33] that the quality of the dataset may impact the classification accuracy. We
explored three key aspects: sentiment polarity, attraction type, and town-wise review instances. This
analysis helped us identify potential issues like class imbalance, which could afect the performance of
the model.</p>
        <sec id="sec-3-1-1">
          <title>3.1.1. Sentiment Polarity</title>
          <p>The dataset is heavily skewed toward positive feedback. As seen in Figure 1, the "Very Positive (5)" label
has the most entries with 136,561 reviews. The "Positive (4)" class follows with 45,034 instances. Neutral
sentiments labeled as "3" are much fewer, totaling 15,519. Negative sentiments are rare—"Negative (2)"
has 5,496 and "Very Negative (1)" has only 5,441 reviews.</p>
          <p>This imbalance means the model might learn to favor positive predictions and ignore the less common
negative or neutral ones. This is a common issue in sentiment datasets and needs to be handled carefully
during pre-processing.</p>
        </sec>
        <sec id="sec-3-1-2">
          <title>3.1.2. Attraction Type</title>
        </sec>
        <sec id="sec-3-1-3">
          <title>3.1.3. Town-wise Distribution</title>
        </sec>
      </sec>
      <sec id="sec-3-2">
        <title>3.2. Mitigating Class Imbalance Issue</title>
        <p>A key challenge in real-world datasets is class imbalance where unequal distribution across target
classes hinders accurate model classification [ 34]. Standard Machine Learning (ML) approaches often
fail on imbalanced data by optimizing majority class accuracy while neglecting minority classes [35].
We used the following three strategies to mitigate the class imbalance issue in the Rest-Mex 2025
training data split.</p>
        <p>Figures 1, 2 and 3 present the distribution on instances based on sentiment polarity, attraction type
and town in the training dataset respectively.</p>
        <sec id="sec-3-2-1">
          <title>3.2.1. Strategy 1: Oversampling</title>
          <p>To mitigate class imbalance issue in the Rest-Mex 2025 training dataset, we used an oversampling
technique to generate synthetic samples for minority classes by randomly duplicating existing instances. For
each underrepresented class, our oversampling technique randomly selects instances (with replacement)
and appends them to the dataset. Then, this oversampled dataset is shufled to ensure randomness
and prevent bias in model training. This approach enhances the representation of minority classes,
improving classifier performance.</p>
        </sec>
        <sec id="sec-3-2-2">
          <title>3.2.2. Strategy 2: Back-translation</title>
          <p>We used the back-translation technique [36] to translate the reviews from the target language to a
source language to enhance the representation of the minority class instances. There are some open
source models available to perform automatic translation of the text. We translated the reviews of
minority class instances in the training dataset to structurally similar target languages (Galician, Italian,
French) and a structurally dissimilar target language (English), and then translated them back to its
source language (Spanish). Three new instances are generated by each instance with label 1 using
the English, French, and Italian languages as target languages. Using back-translation technique, we
added four new instances for each instance having polarity label 1 or 2 and two new instance were
added for each instance with polarity label 3. Table 1 presents the translation models used to perform
back-translation of underrepresented classes in the training dataset along with source language and the
target languages used by these models, and the polarity labels of the instances which were translated
using these models.</p>
        </sec>
        <sec id="sec-3-2-3">
          <title>3.2.3. Strategy 3: Hybrid (Oversampling + Back-translation)</title>
          <p>This strategy combines the previous two strategies (Oversampling and Back-translation) to mitigate the
class imbalance issue. More specifically, we apply the oversampling technique on the dataset created in
strategy 2 using back-translation.</p>
        </sec>
      </sec>
      <sec id="sec-3-3">
        <title>3.3. Model Training</title>
        <p>Transformer models have revolutionized natural language processing (NLP) and sentiment analysis
due to their ability to capture long-range dependencies and contextual relationships in text. In this
model, the representation of each token is computed by attending to all other tokens in the sequence to
capture long-range dependencies regardless of their distance. Unlike recurrent neural networks (RNNs)
or convolutional neural networks (CNNs), transformer models leverage a self-attention mechanism
to weigh the importance of diferent words in a sentence, which enables them to understand nuanced
meanings more efectively [ 37, 38]. This architecture allows transformer models to process entire
sequences in parallel, significantly improving computational eficiency and scalability on large datasets.
Pretrained transformer models, such as BERT [39] and RoBERTa [40], further enhance performance by
leveraging transfer learning enabling fine-tuning on specific tasks like sentiment analysis [39].</p>
        <p>In sentiment analysis, transformer models excel in capturing subtle emotional cues, sarcasm, and
context-dependent expressions. Models like RoBERTa, trained on vast corpora using masked language
modeling, develop a deep understanding of language structure, making them particularly efective
for sentence-level classification tasks [ 40]. Multilingual variants, such as XLM-RoBERTa, extend this
capability across languages, enabling sentiment analysis in diverse linguistic contexts. Platforms like
Huggingface [41] simplify access to these models, providing pretrained implementations that can be
ifne-tuned for domain-specific applications. The transformer models ofer a robust and eficient solution
for sentiment analysis outperforming traditional methods in accuracy and adaptability [39, 38]by
combining contextual awareness with transfer learning.</p>
        <p>For sentiment analysis, we fine-tuned two RoBERTa-based pretrained checkpoints
(roberta-base-bne1 and twitter-xlm-roberta-base2) on Rest-Mex 2025 training datasets
which were created using three strategies to overcome class imbalance issue as explained in section 3.2.
The training dataset was split into a 90:10 ratio for training and validation of the models. Table 2
presents the hyperparameters used to train the models and Table 3 presents the evaluation results of
the trained models on the validation data split and the Table 4 presents the results model evaluation
results on test dataset.</p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>4. Conclusion</title>
      <p>This paper presented a comprehensive framework to address the multifaceted Rest-Mex 2025 challenge,
which involves sentiment polarity prediction, attraction type classification, and specific town
identification from Spanish-language tourist reviews of Mexican Magical Towns. Recognizing the inherent
1https://huggingface.co/PlanTL-GOB-ES/roberta-base-bne
2https://huggingface.co/cardifnlp/twitter-xlm-roberta-base
complexities such as linguistic nuances, informal expressions, and significant class imbalance in the
dataset, we proposed and evaluated several data augmentation strategies.</p>
      <p>Our core contribution lies in the successful application of a hybrid augmentation approach, combining
oversampling with back-translation using both structurally similar and dissimilar languages. This
hybrid strategy, when used to fine-tune transformer-based models, proved highly efective. The results
demonstrate that this approach significantly mitigates the class imbalance issue, leading to improved
performance and generalization across all three subtasks. Notably, the roberta-base-bne model
ifne-tuned with the hybrid dataset achieved an F1 score of 0.9709 for sentiment polarity prediction, and
twitter-xlm-roberta-base also showed strong performance with an F1 score of 0.9694 using the
same strategy.</p>
      <p>The findings underscore the eficacy of targeted data augmentation in enhancing the robustness of
advanced transformer models, particularly in specialized domains like tourism with unique dataset
challenges. Future work could explore the integration of more diverse linguistic resources for
backtranslation, further data augmentation techniques based on paraphrasing, or investigate the adaptability
of these models to other low-resource tourism contexts.</p>
      <p>Our system achieved 4th place in the overall sentiment analysis track of the Rest-Mex 2025 shared
task on sentiment analysis, competing against 35 participating teams. The system obtained an overall
sentiment analysis score of 67.62, with the following detailed performance metrics; polarity prediction
(F1 Macro): 59.92, type prediction (F1 Macro): 98.01 and town prediction (F1 Macro): 62.63. This
competitive ranking demonstrates the robustness of our approach in sentiment classification across
multiple subtasks.</p>
    </sec>
    <sec id="sec-5">
      <title>Acknowledgments</title>
      <p>We acknowledge grants GAP (PID2022-139308OA-I00) funded by MICIU/AEI/10.13039/501100011033/
and ERDF, EU; LATCHING (PID2023-147129OB-C21) funded by MICIU/AEI/10.13039/501100011033
and ERDF, EU; and TSI-100925-2023-1 funded by Ministry for Digital Transformation and Civil Service
and “NextGenerationEU” PRTR; as well as funding by Xunta de Galicia (ED431C 2024/02), and CITIC,
as a center accredited for excellence within the Galician University System and a member of the CIGUS
Network, receives subsidies from the Department of Education, Science, Universities, and Vocational
Training of the Xunta de Galicia. Additionally, it is co-financed by the EU through the FEDER Galicia
2021-27 operational program (Ref. ED431G 2023/01).</p>
    </sec>
    <sec id="sec-6">
      <title>Declaration on Generative AI</title>
      <p>We declare that the present manuscript has been written entirely by the authors and that no generative
artificial intelligence tools were used in its preparation, drafting, or editing.
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