Treebank Explained

In linguistics, a treebank is a parsed text corpus that annotates syntactic or semantic sentence structure. The construction of parsed corpora in the early 1990s revolutionized computational linguistics, which benefitted from large-scale empirical data.^[1]

Etymology

The term treebank was coined by linguist Geoffrey Leech in the 1980s, by analogy to other repositories such as a seedbank or bloodbank.^[2] This is because both syntactic and semantic structure are commonly represented compositionally as a tree structure. The term parsed corpus is often used interchangeably with the term treebank, with the emphasis on the primacy of sentences rather than trees.

Construction

Treebanks are often created on top of a corpus that has already been annotated with part-of-speech tags. In turn, treebanks are sometimes enhanced with semantic or other linguistic information. Treebanks can be created completely manually, where linguists annotate each sentence with syntactic structure, or semi-automatically, where a parser assigns some syntactic structure which linguists then check and, if necessary, correct. In practice, fully checking and completing the parsing of natural language corpora is a labour-intensive project that can take teams of graduate linguists several years. The level of annotation detail and the breadth of the linguistic sample determine the difficulty of the task and the length of time required to build a treebank.

Some treebanks follow a specific linguistic theory in their syntactic annotation (e.g. the BulTreeBank follows HPSG) but most try to be less theory-specific. However, two main groups can be distinguished: treebanks that annotate phrase structure (for example the Penn Treebank or ICE-GB) and those that annotate dependency structure (for example the Prague Dependency Treebank or the Quranic Arabic Dependency Treebank).

It is important to clarify the distinction between the formal representation and the file format used to store the annotated data. Treebanks are necessarily constructed according to a particular grammar. The same grammar may be implemented by different file formats. For example, the syntactic analysis for John loves Mary, shown in the figure on the right, may be represented by simple labelled brackets in a text file, like this (following the Penn Treebank notation):

(S (NP (NNP John)) (VP (VPZ loves) (NP (NNP Mary))) (. .))

This type of representation is popular because it is light on resources, and the tree structure is relatively easy to read without software tools. However, as corpora become increasingly complex, other file formats may be preferred. Alternatives include treebank-specific XML schemes, numbered indentation and various types of standoff notation.

Applications

From a computational linguistics^[3] perspective, treebanks have been used to engineer state-of-the-art natural language processing systems such as part-of-speech taggers, parsers, semantic analyzers and machine translation systems.^[4] Most computational systems utilize gold-standard treebank data. However, an automatically parsed corpus that is not corrected by human linguists can still be useful. It can provide evidence of rule frequency for a parser. A parser may be improved by applying it to large amounts of text and gathering rule frequencies. However, it should be obvious that only by a process of correcting and completing a corpus by hand is it possible then to identify rules absent from the parser knowledge base. In addition, frequencies are likely to be more accurate.

In corpus linguistics, treebanks are used to study syntactic phenomena (for example, diachronic corpora can be used to study the time course of syntactic change). Once parsed, a corpus will contain frequency evidence showing how common different grammatical structures are in use. Treebanks also provide evidence of coverage and support the discovery of new, unanticipated, grammatical phenomena.

Another use of treebanks in theoretical linguistics and psycholinguistics is interaction evidence. A completed treebank can help linguists carry out experiments as to how the decision to use one grammatical construction tends to influence the decision to form others, and to try to understand how speakers and writers make decisions as they form sentences. Interaction research is particularly fruitful as further layers of annotation, e.g. semantic, pragmatic, are added to a corpus. It is then possible to evaluate the impact of non-syntactic phenomena on grammatical choices.

In linguistics research, annotated treebank data has been used in syntactic research to test linguistic theories of sentence structure against large quantities of naturally occurring examples.

Semantic treebanks

A semantic treebank is a collection of natural language sentences annotated with a meaning representation. These resources use a formal representation of each sentence's semantic structure. Semantic treebanks vary in the depth of their semantic representation. A notable example of deep semantic annotation is the Groningen Meaning Bank, developed at the University of Groningen and annotated using Discourse Representation Theory. An example of a shallow semantic treebank is PropBank, which provides annotation of verbal propositions and their arguments, without attempting to represent every word in the corpus in logical form.

Language	Treebank	Semantic Formalism	Distribution / License
Chinese	Chinese Universal Propositions	PropBank semantics
English	Abstract Meaning Representation (AMR) Bank	Deep semantics
English	FrameNet	Shallow semantics
English	Universal Conceptual Cognitive Annotation (UCCA)	Deep semantics
English	Robot Commands Treebank ^[5]	Deep semantics
English	Groningen Meaning Bank	Deep semantics
English	Parallel Meaning Bank	Deep semantics
Dutch	Parallel Meaning Bank	Deep semantics
German	Parallel Meaning Bank	Deep semantics
Italian	Parallel Meaning Bank	Deep semantics
English	DeepBank project	Deep semantics
English	Treebank Semantics Parsed Corpus	Deep semantics
English	RoboCup Corpus	Deep semantics
English	Geoquery	Deep semantics
English	PropBank	PropBank semantics
Finnish	Finnish Universal Propositions	PropBank semantics
Finnish	Finnish PropBank	PropBank semantics
French	French Universal Propositions	PropBank semantics
German	German Universal Propositions	PropBank semantics
Italian	Italian Universal Propositions	PropBank semantics
Portuguese	Portuguese PortLex	PropBank semantics
Portuguese	Portuguese Universal Propositions	PropBank semantics
Spanish	Spanish Universal Propositions	PropBank semantics
Turkish	Turkish PropBank	PropBank semantics

Syntactic treebanks

Many syntactic treebanks have been developed for a wide variety of languages:

Language	Treebank	Syntactic Formalism	Distribution / License
Abaza	Universal Dependencies, ATB	Dependency
Afrikaans	Universal Dependencies, AfriBooms	Dependency
Akkadian	Universal Dependencies, PISANDUB	Dependency
Albanian	Universal Dependencies, TSA	Dependency
Amharic	Universal Dependencies, ATT	Dependency
Ancient Greek	Universal Dependencies, Perseus	Dependency
Ancient Greek	Universal Dependencies, PROIEL	Dependency
Greek (ancient)	Ancient Greek Dependency Treebank^[6] ^[7]	Dependency
Greek (ancient)	PROIEL Treebank^[8]	Dependency
Arabic	Columbia Arabic Treebank (CATiB)	Dependency
Arabic	Prague Arabic Dependency Treebank (PADT)	Dependency
Arabic	Universal Dependencies, NYUAD	Dependency
Arabic	Universal Dependencies, PADT	Dependency
Arabic	Universal Dependencies, PUD	Dependency
Arabic	Penn Arabic Treebank	Phrase structure
Armenian	Universal Dependencies, ArmTDP	Dependency
Assyrian (Neo-Aramaic)	Universal Dependencies, AS	Dependency
Bambara	Universal Dependencies, CRB	Dependency
Basque	Universal Dependencies, BDT	Dependency
Belarusian	Universal Dependencies, HSE	Dependency
Bhojpuri	Universal Dependencies, BhEn	Dependency
Bhojpuri	Universal Dependencies, BHTB	Dependency
Breton	Universal Dependencies, KEB	Dependency
Bulgarian	Universal Dependencies, BTB	Dependency
Bulgarian	BulTreeBank	HPSG
Buryat	Universal Dependencies, BDT	Dependency
Cantonese	Universal Dependencies, HK	Dependency
Catalan	Cat3LB	Phrase structure
Catalan	Universal Dependencies, AnCora	Dependency
Chinese	Sinica Treebank	Case grammar
Chinese	Universal Dependencies, CFL	Dependency
Chinese	Universal Dependencies, GSD	Dependency
Chinese	Universal Dependencies, GSDSimp	Dependency
Chinese	Universal Dependencies, HK	Dependency
Chinese	Universal Dependencies, PUD	Dependency
Chinese	Penn Chinese Treebank	Phrase structure
Chinese	Chinese Dependency Treebank	Dependency
Arabic (classical)	Quranic Arabic Dependency Treebank (QADT) (Quranic Arabic Corpus)	Dependency
Classical Armenian	PROIEL Treebank	Dependency
Coptic	Universal Dependencies, Coptic Scriptorium	Dependency
Croatian	Croatian Dependency Treebank	Dependency
Croatian	Universal Dependencies, SET	Dependency
Czech	Prague Dependency Treebank	Dependency
Czech	Universal Dependencies, CAC	Dependency
Czech	Universal Dependencies, CLTT	Dependency
Czech	Universal Dependencies, FicTree	Dependency
Czech	Universal Dependencies, PDT	Dependency
Czech	Universal Dependencies, PUD	Dependency
Danish	Danish Dependency Treebank	Dependency
Danish	Arboretum: A syntactic tree corpus of Danish	Phrase structure
Danish	Universal Dependencies, DDT	Dependency
Danish	Universal Dependencies, DTB	Dependency
Dutch	Spoken Dutch Corpus (CGN)	Phrase structure
Dutch	Universal Dependencies, Alpino	Dependency
Dutch	Universal Dependencies, LassySmall	Dependency
Dutch	LASSY Small and Large	Dependency
Dutch	Alpino Treebank	Dependency
English	CCGbank	Combinatory categorial grammar
English	LinGO Redwoods	HPSG
English	Lancaster Parsed Corpus	Phrase structure
English	Prague English Dependency Treebank	Dependency
English	Universal Dependencies, BhEn	Dependency
English	Universal Dependencies, ESL	Dependency
English	Universal Dependencies, EWT	Dependency
English	Universal Dependencies, GUM	Dependency
English	Universal Dependencies, GUMReddit	Dependency
English	Universal Dependencies, LinES	Dependency
English	Universal Dependencies, ParTUT	Dependency
English	Universal Dependencies, Pronouns	Dependency
English	Universal Dependencies, PUD	Dependency
English	Treebank Semantics Parsed Corpus	Phrase structure
English	Christine Corpus	Phrase structure
English	Lucy Corpus	Phrase structure
English	Susanne Corpus	Phrase structure
English	BLLIP WSJ corpus	Phrase structure
English	Tübingen Treebank of English / Spontaneous Speech (TüBa-E/S)	HPSG
English	Diachronic Corpus of Present-Day Spoken English (DCPSE)	Phrase structure
English	British Component of the International Corpus of English (ICE-GB)	Phrase structure
English	The PARC 700 Dependency Bank	Dependency
English	Yahoo Query Treebank	Dependency
English	Penn Treebank	Phrase structure
English	Multi-Treebank	Phrase structure
English	CHILDES Brown Eve corpus with dependency annotation	Dependency
English	SMULTRON - Parallel Treebank EN-DE-SV	Phrase structure
Erzya	Universal Dependencies, JR	Dependency
Estonian	Arborest	Phrase structure
Estonian	Syntactically analyzed and disambiguated text corpus	Dependency
Estonian	Universal Dependencies, EDT	Dependency
Estonian	Universal Dependencies, EWT	Dependency
Faroese	Universal Dependencies, FarPaHC	Dependency
Faroese	Universal Dependencies, OFT	Dependency
Finnish	Turku Dependency Treebank (TDT)	Dependency
Finnish	Universal Dependencies, FTB	Dependency
Finnish	Universal Dependencies, PUD	Dependency
Finnish	Universal Dependencies, TDT	Dependency
French (spoken)	Rhapsodie	Dependency and macrosyntactic annotation
French	L'Arboratoire	Phrase structure
French	Universal Dependencies, CrapBank	Dependency
French	Universal Dependencies, FQB	Dependency
French	Universal Dependencies, FTB	Dependency
French	Universal Dependencies, GSD	Dependency
French	Universal Dependencies, ParTUT	Dependency
French	Universal Dependencies, PUD	Dependency
French	Universal Dependencies, Sequoia	Dependency
French	Universal Dependencies, Spoken	Dependency
French	French Treebank	Phrase structure
French	Free French Treebank
French	Sequoia Treebank	Phrase structure & Dependency
Galician	Universal Dependencies, CTG	Dependency
Galician	Universal Dependencies, TreeGal	Dependency
German	Hamburg Dependency Treebank (HDT)	Dependency
German	Universal Dependencies, GSD	Dependency
German	Universal Dependencies, LIT	Dependency
German	Universal Dependencies, PUD	Dependency
German	SMULTRON - Parallel Treebank EN-DE-SV	Phrase structure
German	NEGRA	Phrase structure
German	TIGER	Phrase structure
German	Tübingen Treebank of German / Spontaneous Speech (TüBa-D/S)	Phrase structure
German	Tübingen Treebank of Written German (TüBa-D/Z)	Phrase structure
German	Tübingen Partially Parsed Corpus of Written German (TüPP-D/Z)	Phrase structure
Gothic	PROIEL Treebank	Dependency
Gothic	Universal Dependencies, PROIEL	Dependency
Greek	Greek Dependency Treebank	Dependency
Greek	Universal Dependencies, GDT	Dependency
Hebrew	Universal Dependencies, HTB	Dependency
Hebrew	Hebrew Dependency Treebank	Dependency
Hindi English	Universal Dependencies, HIENCS	Dependency
Hindi	Universal Dependencies, HDTB	Dependency
Hindi	Universal Dependencies, PUD	Dependency
Hindi	AnnCorra	Dependency
English (historical)	Penn Parsed Corpora of Historical English	Phrase structure
English (historical)	York-Toronto-Helsinki Parsed Corpus of Old English Prose (YCOE)	Phrase structure
French (historical)	Corpus MCVF	Phrase structure
Portuguese (historical)	Tycho Brahe corpus	Phrase structure
Hungarian	Universal Dependencies, Szeged	Dependency
Hungarian	Hungarian Treebank	Phrase structure
Icelandic	IcePaHC - Icelandic Parsed Historical Corpus	Phrase structure
Icelandic	Universal Dependencies, IcePaHC	Dependency
Icelandic	Universal Dependencies, PUD	Dependency
Indonesian	Universal Dependencies, GSD	Dependency
Indonesian	Universal Dependencies, PUD	Dependency
Indonesian	ICON	Phrase structure
Irish	Universal Dependencies, IDT	Dependency
Italian	ISST - Italian Syntactic-Semantic Treebank	Phrase structure and dependency
Italian	MIDT (Merged Italian Dependency Treebank) resulting from the merging and harmonization of the TUT and ISST-CoNLL/TANL treebanks	dependency
Italian	VIT - Venice Italian Treebank	Phrase structure and dependency
Italian	Universal Dependencies, ISDT	Dependency
Italian	Universal Dependencies, ParTUT	Dependency
Italian	Universal Dependencies, PoSTWITA	Dependency
Italian	Universal Dependencies, PUD	Dependency
Italian	Universal Dependencies, TWITTIRO	Dependency
Italian	Universal Dependencies, VIT	Dependency
Italian	Italian Syntactic-Semantic Treebank for the CoNLL-2007 Shared Task (ISST-CoNLL)	dependency
Italian	SUT - Siena University Treebank
Italian	TUT - Turin University Treebank	Dependency
Italian	ISDT (Italian Stanford Dependency Treebank)	dependency
Japanese	Kyoto Text Corpus
Japanese	Universal Dependencies, BCCWJ	Dependency
Japanese	Universal Dependencies, GSD	Dependency
Japanese	Universal Dependencies, KTC	Dependency
Japanese	Universal Dependencies, Modern	Dependency
Japanese	Universal Dependencies, PUD	Dependency
Japanese	Keyaki Treebank	Phrase structure
Japanese	Tübingen Treebank of Japanese / Spontaneous Speech (TüBa-J/S)	Phrase structure
Japanese	ATR Dependency corpus	Dependency
Karelian	Universal Dependencies, KKPP	Dependency
Kazakh	Universal Dependencies, KTB	Dependency
Komi Permyak	Universal Dependencies, UH	Dependency
Komi Zyrian	Universal Dependencies, IKDP	Dependency
Komi Zyrian	Universal Dependencies, Lattice	Dependency
Korean	Universal Dependencies, GSD	Dependency
Korean	Universal Dependencies, Kaist	Dependency
Korean	Universal Dependencies, Penn	Dependency
Korean	Universal Dependencies, PUD	Dependency
Korean	Universal Dependencies, Sejong	Dependency
Korean	Korean Treebank	Phrase structure
Kurmanji	Universal Dependencies, MG	Dependency
Latin	Universal Dependencies, ITTB	Dependency
Latin	Universal Dependencies, LLCT	Dependency
Latin	Universal Dependencies, Perseus	Dependency
Latin	Universal Dependencies, PROIEL	Dependency
Latin	Index Thomisticus Treebank	Dependency
Latin	PROIEL Treebank	Dependency
Latin	Latin Dependency Treebank^[9]	Dependency
Latvian	Universal Dependencies, LVTB	Dependency
Lithuanian	Universal Dependencies, ALKSNIS	Dependency
Lithuanian	Universal Dependencies, HSE	Dependency
Livvi	Universal Dependencies, KKPP	Dependency
Magahi	Universal Dependencies, MGTB	Dependency
Maltese	Universal Dependencies, MUDT	Dependency
Marathi	Universal Dependencies, UFAL	Dependency
Mbya Guarani	Universal Dependencies, Dooley	Dependency
Mbya Guarani	Universal Dependencies, Thomas	Dependency
Middle Irish	Universal Dependencies, CritMITB	Dependency
Middle Irish	Universal Dependencies, DipMITB	Dependency
Moksha	Universal Dependencies, JR	Dependency
Naija	Universal Dependencies, NSC	Dependency
North Sami	Universal Dependencies, Giella	Dependency
Norwegian	INESS treebanking infrastructure	LFG
Norwegian	Universal Dependencies, Bokmaal	Dependency
Norwegian	Universal Dependencies, Nynorsk	Dependency
Norwegian	Universal Dependencies, NynorskLIA	Dependency
Old Church Slavonic	Universal Dependencies, PROIEL	Dependency
Old Church Slavonic	TOROT Treebank	Dependency
Old French	Universal Dependencies, SRCMF	Dependency
Old Russian	Universal Dependencies, RNC	Dependency
Old Russian	Universal Dependencies, TOROT	Dependency
Old Russian	TOROT Treebank	Dependency
Persian	Persian Dependency Treebank (PerDT)	Dependency
Persian	PerTreeBank	HPSG
Persian	Universal Dependencies, Seraji	Dependency
Polish	A Treebank / Test Suite for Polish	HPSG
Polish	Universal Dependencies, LFG	Dependency
Polish	Universal Dependencies, PDB	Dependency
Polish	Universal Dependencies, PUD	Dependency
Polish	Składnica	Phrase structure and Dependency
Portuguese	Universal Dependencies, Bosque	Dependency
Portuguese	Universal Dependencies, GSD	Dependency
Portuguese	Universal Dependencies, PUD	Dependency
Portuguese	Projecto Floresta Sintá(c)tica	Dependency, Phrase structure
Romanian	Romanian Dependency Treebank	Dependency
Romanian	Universal Dependencies, Nonstandard	Dependency
Romanian	Universal Dependencies, RRT	Dependency
Romanian	Universal Dependencies, SiMoNERo	Dependency
Russian	Universal Dependencies, GSD	Dependency
Russian	Universal Dependencies, PUD	Dependency
Russian	Universal Dependencies, SynTagRus	Dependency
Russian	Universal Dependencies, Taiga	Dependency
Russian	SynTagRus Dependency Treebank (Russian National Corpus)	Dependency
Sanskrit	Universal Dependencies, UFAL	Dependency
Sanskrit	Universal Dependencies, Vedic	Dependency
Scottish Gaelic	Universal Dependencies, ARCOSG	Dependency
Serbian	Universal Dependencies, SET	Dependency
Sindhi	Universal Dependencies, MazharDootio	Dependency
Skolt Sami	Universal Dependencies, Giellagas	Dependency
Slovak	Universal Dependencies, SNK	Dependency
Slovene	Slovene Dependency Treebank	Dependency
Slovenian	Universal Dependencies, SSJ	Dependency
Slovenian	Universal Dependencies, SST	Dependency
Spanish	Cast3LB	Phrase structure and dependency
Spanish	Universal Dependencies, AnCora	Dependency
Spanish	Universal Dependencies, GSD	Dependency
Spanish	Universal Dependencies, PUD	Dependency
Spanish	UAM Treebank of Spanish	Phrase structure
Swedish	Talbanken05	Phrase structure and dependency
Swedish	Swedish Treebank	Phrase structure
Swedish	Universal Dependencies, LinES	Dependency
Swedish	Universal Dependencies, PUD	Dependency
Swedish	Universal Dependencies, Talbanken	Dependency
Swedish	SMULTRON - Parallel Treebank EN-DE-SV	Phrase structure
Swedish Sign Language	Universal Dependencies, SSLC	Dependency
Swiss German	Universal Dependencies, UZH	Dependency
Tagalog	Universal Dependencies, TRG	Dependency
Tagalog	Universal Dependencies, Ugnayan	Dependency
Tamil	Universal Dependencies, TTB	Dependency
Telugu	Universal Dependencies, MTG	Dependency
Thai	NAiST Thai Treebank	Dependency
Thai	Universal Dependencies, PUD	Dependency
Thai	THTB	Phrase structure
Turkish	METU-Sabanci Turkish Treebank	Dependency
Turkish	Universal Dependencies, BOUN	Dependency
Turkish	Universal Dependencies, GB	Dependency
Turkish	Universal Dependencies, IMST	Dependency
Turkish	Universal Dependencies, PUD	Dependency
Ukrainian	Institute for Ukrainian, NGO Gold Standard	Dependency
Ukrainian	Universal Dependencies, IU	Dependency
Upper Sorbian	Universal Dependencies, UFAL	Dependency
Urdu	NU-FAST Treebank	Phrase structure
Urdu	The URDU.KON-TB Treebank	Phrase and Hyper Dependency Structure
Urdu	Universal Dependencies, UDTB	Dependency
Uyghur	Universal Dependencies, UDT	Dependency
Vietnamese	Universal Dependencies, VTB	Dependency
Vietnamese	Vietnamese Treebank	Phrase structure
Vietnamese	Vietnamese Dependency Treebank	Dependency
Warlpiri	Universal Dependencies, UFAL	Dependency
Welsh	Universal Dependencies, CCG	Dependency
Wolof	Universal Dependencies, WTB	Dependency
Yoruba	Universal Dependencies, YTB	Dependency

To facilitate the further researches between multilingual tasks, some researchers discussed the universal annotation scheme for cross-languages. In this way, people try to utilize or merge the advantages of different treebanks corpora. For instance, The universal annotation approach for dependency treebanks;^[10] and the universal annotation approach for phrase structure treebanks.^[11]

Search tools

One of the key ways to extract evidence from a treebank is through search tools. Search tools for parsed corpora typically depend on the annotation scheme that was applied to the corpus. User interfaces range in sophistication from expression-based query systems aimed at computer programmers to full exploration environments aimed at general linguists. Wallis (2008) discusses the principles of searching treebanks in detail and reviews the state of the art around that time.^[12]

See also

• Text corpus
• Phrase structure grammar
• Dependency grammar
• Parsing
• Part-of-speech tagging

Notes and References

1. Alexander Clark, Chris Fox and Shalom Lappin (2010). The handbook of computational linguistics and natural language processing. Wiley.
2. Sampson, G. (2003) ‘Reflections of a dendrographer.’ In A. Wilson, P. Rayson and T. McEnery (eds.) Corpus Linguistics by the Lune: A Festschrift for Geoffrey Leech, Frankfurt am Main: Peter Lang, pp. 157-184
3. Haitao Liu, Wei Huang — A Chinese Dependency Syntax for Treebanking, published by Communication University of China, published (online) by the Association for Computational Linguistics - accessed 2020-2-4
4. Kübler. Sandra. McDonald. Ryan. Nivre. Joakim. 2008-12-18. Dependency Parsing. Synthesis Lectures on Human Language Technologies. en. 2. 1. 1–127. 10.2200/s00169ed1v01y200901hlt002.
5. Kais Dukes (2013) Semantic Annotation of Robotic Spatial Commands. Language and Technology Conference (LTC). Poznan, Poland.
6. Celano, Giuseppe G. A. 2014. Guidelines for the annotation of the Ancient Greek Dependency Treebank 2.0. https://github.com/PerseusDL/treebank_data/edit/master/AGDT2/guidelines
7. Mambrini, F. 2016. The Ancient Greek Dependency Treebank: Linguistic Annotation in a Teaching Environment. In: Bodard, G & Romanello, M (eds.) Digital Classics Outside the Echo-Chamber: Teaching, Knowledge Exchange & Public Engagement, Pp. 83–99. London: Ubiquity Press.
8. Dag Haug. 2015. Treebanks in historical linguistic research. In Carlotta Viti (ed.), Perspectives on Historical Syntax, Benjamins, 188-202. A preprint is available at http://folk.uio.no/daghaug/historical-treebanks.pdf.
9. Bamman David & al. 2008. Guidelines for the Syntactic Annotation of Latin Treebanks (v. 1.3). http://nlp.perseus.tufts.edu/syntax/treebank/1.3/docs/guidelines.pdf
10. McDonald, R. . Nivre, J., Quirmbach-Brundage, Y. . Universal Dependency Annotation for Multilingual Parsing.. Proceedings of the ACL 2013. . etal.
11. Han, A.L.-F . Wong, D.F. . Chao, L.S. . Lu, Y. . He, L. . Tian, L. . amp . A Universal Phrase Tagset for Multilingual Treebanks. Proceedings of the CCL and NLP-NABD 2014, LNAI 8801, pp. 247– 258. © Springer International Publishing Switzerland . 2014. 10.1007/978-3-319-12277-9_22 .
12. Wallis, Sean (2008). Searching treebanks and other structured corpora. Chapter 34 in Lüdeling, A. & Kytö, M. (ed.) Corpus Linguistics: An International Handbook. Handbücher zur Sprache und Kommunikationswissenschaft series. Berlin: Mouton de Gruyter.