Interactive grammar development with WCDG
Kilian A. Foth Michael Daum Wolfgang Menzel
Natural Language Systems Group
Hamburg University
D-22527 Hamburg
Germany
ffoth,micha,menzelg@nats.informatik.uni-hamburg.de
Abstract
The manual design of grammars for accurate natu-
ral language analysis is an iterative process; while
modelling decisions usually determine parser be-
haviour, evidence from analysing more or differ-
ent input can suggest unforeseen regularities, which
leads to a reformulation of rules, or even to a differ-
ent model of previously analysed phenomena. We
describe an implementation of Weighted Constraint
Dependency Grammar that supports the grammar
writer by providing display, automatic analysis, and
diagnosis of dependency analyses and allows the di-
rect exploration of alternative analyses and their sta-
tus under the current grammar.
1 Introduction
For parsing real-life natural language reliably, a
grammar is required that covers most syntactic
structures, but can also process input even if it
contains phenomena that the grammar writer has
not foreseen. Two fundamentally different ways
of reaching this goal have been employed various
times. One is to induce a probability model of the
target language from a corpus of existing analyses
and then compute the most probable structure for
new input, i.e. the one that under some judiciously
chosen measure is most similar to the previously
seen structures. The other way is to gather linguis-
tically motivated general rules and write a parsing
system that can only create structures adhering to
these rules.
Where an automatically induced grammar re-
quires large amounts of training material and the
development focuses on global changes to the prob-
ability model, a handwritten grammar could in prin-
ciple be developed without any corpus at all, but
considerable effort is needed to  nd and formu-
late the individual rules. If the formalism allows
the ranking of grammar rules, their relative impor-
tance must also be determined. This work is usu-
ally much more cyclical in character; after grammar
rules have been changed, intended and unforeseen
consequences of the change must be checked, and
further changes or entirely new rules are suggested
by the results.
We present a tool that allows a grammar writer to
develop and re ne rules for natural language, parse
new input, or annotate corpora, all in the same envi-
ronment. Particular support is available for interac-
tive grammar development; the effect of individual
grammar rules is directly displayed, and the system
explicitly explains its parsing decisions in terms of
the rules written by the developer.
2 The WCDG parsing system
The WCDG formalism (Schrcurrency1oder, 2002) describes
natural language exclusively as dependency struc-
ture, i.e. ordered, labelled pairs of words in the in-
put text. It performs natural language analysis under
the paradigm of constraint optimization, where the
analysis that best conforms to all rules of the gram-
mar is returned. The rules are explicit descriptions
of well-formed tree structures, allowing a modular
and  ne-grained description of grammatical knowl-
edge. For instance, rules in a grammar of English
would state that subjects normally precede the  nite
verb and objects follow it, while temporal NP can
either precede or follow it.
In general, these constraints are defeasible, since
many rules about language are not absolute, but
can be preempted by more important rules. The
strength of constraining information is controlled by
the grammar writer: fundamental rules that must al-
ways hold, principles of different import that have
to be weighed against each other, and general pref-
erences that only take effect when no other disam-
biguating knowledge is available can all be formu-
lated in a uniform way. In some cases preferences
can also be used for disambiguation by approximat-
ing information that is currently not available to the
system (e.g. knowledge on attachment preferences).
Even the very weak preferences have an in uence
on the parsing process; apart from serving as tie-
breakers for structures where little context is avail-
able (e.g. with fragmentary input), they provide an
Figure 1: Display of a simpli ed feature hierarchy
initial direction for the constraint optimization pro-
cess even if they are eventually overruled. As a con-
sequence, even the best structure found usually in-
curs some minor constraint violations; as long as
the combined evidence of these default expectation
failures is small, the structure can be regarded as
perfectly grammatical.
The mechanism of constraint optimization si-
multaneously achieves robustness against extra-
grammatical and ungrammatical input. There-
fore WCDG allows for broad-coverage parsing with
high accuracy; it is possible to write a grammar
that is guaranteed to allow at least one structure for
any kind of input, while still preferring compliant
over deviant input wherever possible. This graceful
degradation under reduced input quality makes the
formalism suitable for applications where deviant
input is to be expected, e.g. second language learn-
ing. In this case the potential for error diagnosis
is also very valuable: if the best analysis that can
be found still violates an important constraint, this
directly indicates not only where an error occurred,
but also what might be wrong about the input.
3 XCDG: A Tool for Parsing and
Modelling
An implementation of constraint dependency gram-
mar exists that has the character of middleware to al-
low embedding the parsing functionality into other
natural language applications. The program XCDG
uses this functionality for a graphical tool for gram-
mar development.
In addition to providing an interface to a range
of different parsing algorithms, graphical display
of grammar elements and parsing results is possi-
ble; for instance, the hierarchical relations between
possible attributes of lexicon items can be shown.
See Figure 1 for an excerpt of the hierarchy of Ger-
man syntactical categories used; the terminals cor-
respond to those used the Stuttgart-Tcurrency1ubingen Tagset
of German (Schiller et al., 1999).
More importantly, mean and end results of pars-
ing runs can be displayed graphically. Dependency
structures are represented as trees, while additional
relations outside the syntax structure are shown as
arcs below the tree (see the referential relationship
REF in Figure 2). As well as end results, inter-
mediate structures found during parsing can be dis-
played. This is often helpful in understanding the
behaviour of the heuristic solution methods em-
ployed.
Together with the structural analysis, instances
of broken rules are displayed below the depen-
dency graph (ordered by decreasing weights), and
the dependencies that trigger the violation are high-
lighted on demand (in our case the PP-modi cation
between the preposition in and the in nite form
verkaufen). This allows the grammar writer to eas-
ily check whether or not a rule does in fact make the
distinction it is supposed to make. A unique iden-
ti er attached to each rule provides a link into the
grammar source  le containing all constraint de -
nitions. The unary constraint ’mod-Distanz’ in
the example of Figure 2 is a fairly weak constraint
which penalizes attachments the stronger the more
distant a dependent is placed from its head. At-
taching the preposition to the preceding noun Bund
would be preferred by this constraint, since the dis-
tance is shorter. However, it would lead to a more
serious constraint violation because noun attach-
ments are generally dispreferred.
To facilitate such experimentation, the parse win-
dow doubles as a tree editor that allows structural,
lexical and label changes to be made to an analysis
by drag and drop. One important application of the
integrated parsing and editing tool is the creation of
large-scale dependency treebanks. With the ability
to save and load parsing results from disk, automat-
ically computed analyses can be checked and hand-
corrected where necessary and then saved as anno-
tations. With a parser that achieves a high perfor-
mance on unseen input, a throughput of over 100 an-
notations per hour has been achieved.
4 Grammar development with XCDG
The development of a parsing grammar based on
declarative constraints differs fundamentally from
that of a derivational grammar, because its rules for-
bid structures instead of licensing them: while a
context-free grammar without productions licenses
nothing, a constraint grammar without constraints
would allow everything. A new constraint must
therefore be written whenever two analyses of the
same string are possible under the existing con-
straints, but human judgement clearly prefers one
over the other.
Figure 2: Xcdg Tree Editor
Most often, new constraints are prompted by in-
spection of parsing results under the existing gram-
mar: if an analysis is computed to be grammati-
cal that clearly contradicts intuition, a rule must be
missing from the grammar. Conversely, if an error
is signalled where human judgement disagrees, the
relevant grammar rule must be wrong (or in need of
clarifying exceptions). In this way, continuous im-
provement of an existing grammar is possible.
XCDG supports this development style through
the feature of hypothetical evaluation. The tree dis-
play window does not only show the result returned
by the parser; the structure, labels and lexical selec-
tions can be changed manually, forcing the parser to
pretend that it returned a different analysis. Recall
that syntactic structures do not have to be specif-
ically allowed by grammar rules; therefore, every
conceivable combination of subordinations, labels
and lexical selections is admissible in principle, and
can be processed by XCDG, although its score will
be low if it contradicts many constraints.
After each such change to a parse tree, all con-
straints are automatically re-evaluated and the up-
dated grammar judgement is displayed. In this way
it can quickly be checked which of two alternative
structures is preferred by the grammar. This is use-
ful in several ways. First, when analysing pars-
ing errors it allows the grammar author to distin-
guish search errors from modelling errors: if the
intended structure is assigned a better score than the
one actually returned by the parser, a search error
occurred (usually due to limited processing time);
but if the computed structure does carry the higher
score, this indicates an error of judgement on the
part of the grammar writer, and the grammar needs
to be changed in some way if the phenomenon is to
be modelled adequately.
If a modelling error does occur, it must be be-
cause a constraint that rules against the intended
analysis has overruled those that should have se-
lected it. Since the display of broken constraints is
ordered by severity, it is immediately obvious which
of the grammar rules this is. The developer can
then decide whether to weaken that rule or extend
it so that it makes an exception for the current phe-
nomenon. It is also possible that the intended anal-
ysis really does con ict with a particular linguistic
principle, but in doing so follows a more important
one; in this case, this other rule must be found and
strengthened so that it will overrule the  rst one.
The other rule can likewise be found by re-creating
the original automatic analysis and see which of its
constraint violations needs to be given more weight,
or, alternatively, which entirely new rule must be
added to the grammar.
In the decision whether to add a new rule to a con-
straint grammar, it must be discovered under what
conditions a particular phenomenon occurs, so that
a generally relevant rule can be written. The posses-
sion of a large amount of analysed text is often use-
ful here to verify decisions based on mere introspec-
tion. Working together with an external program
to search for speci c structures in large treebanks,
XCDG can display multiple sentences in stacked
widgets and highlight all instances of the same phe-
nomenon to help the grammar writer decide what
the relevant conditions are.
Using this tool, a comprehensive grammar of
modern German has been constructed (Foth, 2004)
that employs 750 handwritten well-formedness
rules, and has been used to annotate around 25,000
sentences with dependency structure. It achieves a
structural recall of 87.7% on sentences from the NE-
GRA corpus (Foth et al., submitted), but can be ap-
plied to texts of many other types, where structural
recall varies between 80 90%. To our knowledge,
no other system has been published that achieves
a comparable correctness for open-domain German
text. Parsing time is rather high due to the computa-
tional effort of multidimensional optimization; pro-
cessing time is usually measured in seconds rather
than milliseconds for each sentence.
5 Conclusions
We demonstrate a tool that lets the user parse, dis-
play and manipulate dependency structures accord-
ing to a variant of dependency grammar in a graph-
ical environment. We have found such an inte-
grated environment invaluable for the development
of precise and large grammars of natural language.
Compared to other approaches, c.f. (Kaplan and
Maxwell, 1996), the built-in WCDG parser pro-
vides a much better feedback by pinpointing possi-
ble reasons for the current grammar being unable to
produce the desired parsing result. This additional
information can then be immediately used in subse-
quent development cycles.
A similar tool, called Annotate, has been de-
scribed in (Brants and Plaehn, 2000). This tool
facilitates syntactic corpus annotation in a semi-
automatic way by using a part-of-speech tagger and
a parser running in the background. In compari-
son, Annotate is primarily used for corpus annota-
tion, whereas XCDG supports the development of
the parser itself also.
Due to its ability to always compute the single
best analysis of a sentence and to highlight possible
shortcomings of the grammar, the XCDG system
provides a useful framework in which human design
decisions on rules and weights can be effectively
combined with a corpus-driven evaluation of their
consequences. An alternative for a symbiotic coop-
eration in grammar development has been devised
by (Hockenmaier and Steedman, 2002), where a
skeleton of fairly general rule schemata is instan-
tiated and weighed by means of a treebank anno-
tation. Although the resulting grammar produced
highly competitive results, it nevertheless requires
a treebank being given in advance, while our ap-
proach also supports a simultaneous treebank com-
pilation.
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