Eckhard Bick - VISL

For global context syntactic rules (i.e. rules containing at least one unbounded
context condition), selecting is even more risky than ordinarily. For example, it is safe
to assume that a direct object reading can be removed in a sentence without a transitive
verb, while the "inverse", choosing the object reading in the presence of a transitive
verb, is risky, rules would have to thoroughly check for other direct objects and direct
object candidates, clause boundaries and the like.
In the case of tag targeting rules, cautiousness is necessary because a tag-target
has to cover a range of possibly quite different lexical items, whereas a word-form
target is really the equivalent of a complete tag sequence, including the lexical base
form tag. Since very few syntactic rules have word-form targets, the effect is only
visible in the morphological rule portion, with a remove/select ratio of 1.5 for tag
targeting rules as compared to one of 1.0 for morphological rules on a whole.
The elevated remove/select ratio for syntactic rules (over 4) is not only due to a
higher degree of "structural globality" (as addressed by the valency based uniqueness
principle), but also to the grammar specific fact that clause function tags have been
attached to non-finite verbs and complementiser words (relatives, interrogatives,
conjunctions), in addition to these words' clause internal function tag. Since double tag
targets are not allowed in syntactic SELECT rules in the available cg-compilers, such
words can only be disambiguated by REMOVE rules - a SELECT rule targeted at either
the internal or the external function tag would "kill" the other of the two.
Apart from low error rates, Constraint Grammar parsers are famous for their processing
speed. The actual speed, even when using the same compiler on the same machine, is of
course dependent on both text type and grammar size. For text type, the relevant
parameters are sentence length and word form ambiguity (average number of readings
per word form); for grammar size, parameters are the number of rule contexts
(subsuming the number of rules as well as their complexity) and the proportion of
unbounded contexts. Since the parser has to apply for every word and every one of its
readings all rules that target that reading, a first approximation for sentence processing
time would be one of linear complexity:
(3a) time ~ n * a * R
where
n = number of words in the sentence
a = average ambiguity (number of readings per word form
R = rule number constant, depending on, but less than proportional to the number
of rules Rn in the grammar
However, since the parser - in applying a rule to the target reading found - must check
("instantiate") all the rule's context conditions as true, the relevant constant is not the
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