Zero Morphemes in Unification-Based Combinatory Categorial Grammar

In this paper, we report on our use of zero morphemes in Unification-Based Combinatory Categorial Grammar. After illustrating the benefits of this approach with several examples, we describe the algorithm for compiling zero morphemes into unary rules, which allows us to use zero morphemes more efficiently in natural language processing. 1 Then, we discuss the question of equivalence of a grammar with these unary rules to the original grammar. Lastly, we compare our approach to zero morphemes with possible alternatives. 1. Zero Morphemes in Categorial Grammar In English and in other natural languages, it is attractive to posit the existence of morphemes that are invisible on the surface but have their own syntactic and semantic definitions. In our analyses, they are just like any other overt morphemes except for having null strings (i.e. " "), and we call them zero morphemes. Most in Categorial Grammar and related forms of unification-based grammars, on the other hand, take the rule-based approach. That is, they assume that there are unary rules that change features or categories of their arguments (cf. Dowty 1977, Hoeksema 1985, Wittenburg 1986, Wood 1987). Below, we will discuss the advantages of our zero morpheme approach over the rule-based approach. Zero morphemes should be distinguished from so-called "gaps" in wh-questions and relative clauses in that zero morphemes are not traces or "place holders" of any other overt morphemes in a given sentence. There are at 1. The work described here is implemented in Common Lisp and being used in the Lucy natural language understanding system at MCC. 188 least two types of zero morphemes: zero morphemes at the morphology level and those at the syntax level. A zero morpheme at the morphology level applies to a free morpheme and forms an inflected word. Such examples are present tense zero morpheme (PRES) as in 'I like+PRES dogs" and a singular zero morpheme (SG) as in "a dog+SG". These two are the counterparts of a third person singular present tense morpheme C+s" as in "John like+s dogs" and a plural morpheme C+s" as in 'two dog+s'~, respectively. (1) dog +SG N[num:null] N[num:sg]~N[num:null] dog +s N[num:null] N[num:pl]\N[num:null] Notice that, unlike the rule-based approach, the declarative and compositional nature of the zero morpheme approach makes the semantic analysis easier, since each zero morpheme has its semantic definition in the lexicon and therefore can contribute its semantics to the whole interpretation just as an overt morpheme does. Also, the monotonicity of our 'feature adding" approach, as opposed to "default feature" approach (e.g., Gazdar 1987), is attractive in compositional semantics because it does not have to retract or override a semantic translation contributed by a word with a default feature. For example, "dog" in both "dog+SG" and "dog+s" contributes the same translation, and the suffixes "+SG" and "+s" just add the semantics of number to their respective head nouns. In addition, this approach helps reduce redundancy in the lexicon. For instance, we do not have to define for each base verb in the lexicon their presenttense counterparts. a man REL-MOD the daughter of whom N (N\N)/S[reI:+] NP/N N (N\N)/NP NP[rel:+] apply> . . . . . N[rel:+]\N apply< . . . . N[rel:+] I apply> ,, NP[rel:+] LIFT S[reh+]/(S/NP) apply> S[rel:+] apply> N\N John liked NP (S\NP)/NP ~p~pera i s ing