Eckhard Bick - VISL

ambiguity, Constraint Grammar can also be used for syntactic parsing, as efficiently
shown, for instance, in the Bank-of-English-project (200 million words, Järvinen,
1994).
(3) Input to the syntactic CG-rules (after mapping)
""
"nunca" ADV @ADVL
""
"como" V PR 1S VFIN @FMV
""
"peixe" N M S @SUBJ @ACC @SC @OC
[@ADVL=adverbial, @FMV=finite main verb, @SUBJ=subject, @ACC=direct object, @SC=subject complement,
@OC=object complement]
In (3), adding all possible syntactic tags (@) has resulted in fourfold syntactic
ambiguity for peixe. The direct object reading (@ACC) can be selected in a positive
way by means of a 'SELECT'- rule exploiting the transitivity of the verb, but it could
just as well be identified indirectly, - by being the only surviving reading, after CGrules
have discarded all others:
(4) REMOVE (@SUBJ) IF (0 N) (NOT *-1 V3) (NOT *1 V3)
[discard the subject reading, if the target is a noun (N) and there is no verb in the 3.person]
REMOVE (@SC) IF (NOT *-1 ) (NOT *1 )
[discard the subject complement reading (@SC) if there is no copula verb () in the sentence]
REMOVE (@OC) IF (NOT *-1 @ACC) (NOT *1 @ACC)
[discard the object complement reading (@OC) if there is no direct object reading (@ACC) in the sentence] 104
It is this indirect disambiguation, that is most characteristic of Constraint Grammar, and
it is the prime reason for the robustness of this method: even rare or incomplete
constructions will receive at least one reading - the one that survives the most
constraints. The incremental use of the rules, with safe contexts and safe rules before
ambiguous contexts and heuristic rules, furthermore ensures that the parser will prefer a
reading that is "almost correct" to one that is "quite wrong".
CG-grammars have first of all been described for English (e.g. Karlsson et.al.,
1991), but there are - on the morphological level, at least - projects involving several
103 In the mapping modul, constraint grammar rules are used, too, and the list of possible syntactic functions for a given word
form can thus be made context dependent (and, of course, shorter).
104 Note that all 3 rules make use of "unbound" contexts conditions:
*-1 = the context condition is to be true anywhere to the left (1 or more positions to the left)
*1 = the context condition is to be true anywhere to the right (1 or more positions to the right)
Of course,"bound" context conditions can be used, e.g. -2 = second word to the left, 3 = third word to the right. Bound
context conditions can in principle be translated into n-gram rules (as used in probabilistic HMM parsers), while "unbound"
(*-context) conditions are characteristic of Constraint Grammar and not easily translatable into probabilistic systems (cp.
also chapter 3.7.3).
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