Phrasal Analysis of Long Noun Sequences

Noun phrases consisting of a sequence of nouns (sometimes referred to as nominal compounds) pose considerable difficulty for language analyzers but are common in many technical domains. The problems are compounded when some of the nouns in the sequence are ambiguously also verbs. The phrasal approach to language analysis, as implemented in PHRAN (PHRasal ANalyzer), has been extended to handle the recognition and partial analysis of such constructions. The phrasal analysis of a noun sequence is performed to an extent sufficient for continued analysis of the sentence in which it appears. PHRAN is currently being used as part of the SPAN (SPecification ANalysis) natural language interface to the USC Advanced Design AutoMation system (ADAM) (Granacki ct at, 1985). PHRA_N-SPAN is an interface for entering and interpreting digital system specifications, in which long noun sequences occur often. The extensions to PHRAN's knowledge base to recognize these constructs are described, along with the algorithm used to detect and resolve ambiguities which arise in the noun sequences. 1. I n t r o d u c t i o n In everyday language we routinely encounter noun phrases consisting of an article and a head noun, possibly modified by one or more adjectives. Noun-noun pairs, e.g., park bench, atom bomb, and computer programmer, are also common. It is rare, however, to encounter noun phrases consisting of three or more nouns in sequence. Consequently, research in natural language analysis has not concentrated on parsing such constructions. The situation in many technical fields is quite different. For example, when describing the specifications of electronic systems, designers commonly use expressions such as: bus request cycle transfer block size segment trap request interrupt vector transfer phase arithmetic register transfer instruction. During design specification such phrases are often constructed by the specifier in order to reference a particular entity: a piece of hardware, an activity, or a range of time. In most cases, the nouns preceding the last one are used as modifiem, and idiomatic expressions are very rare. In almost all cases the meaning of noun sequences can therefore be inferred largely based on the last noun in the sequence*. (But see Finin (1980) for in-depth treatment of the meaning of such constructions). The process of recognizing the presence of these expressions is, however, complicated by the fact that many of the words used are syntactically ambiguous. Almost every single word used in the examples above belongs to both the syntactic categories of noun and verb. As a result,