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The Hearsay II speech understanding system being developed at Carnegie-Mellon University has an independent knowledge source module for each type of speech knowledge. Modules communicate by reading, writing, and modifying hypotheses about various constituents of the spoken utterance in a global data structure. The syntax and semantics module uses rules (productions) of four types: (1) recognition rules for generating a phrase hypothesis when its needed constituents have already been hypothesized; (2) prediction rules for inferring the likely presence of a word or phrase from previously recognized portions of the utterance; (3) respelling rules for hypothesizing the constituents of a predicted phrase; and (4) postdiction rules for supporting an existing hypothesis on the basis of additional confirming evidence. The rules are automatically generated from a declarative (i.e., non-procedural) description of the grammar and semantics, and are embedded in a parallel recognition network for efficient retrieval of applicable rules. The current grammar uses a 450- word vocabulary and accepts simple English queries for an information retrieval system. (Author)