An Efficient Easily Adaptable System for Interpreting Natural Language Queries 
 
 This paper gives an overall account of a prototype natural language question answering system , called Chat-80 . Chat-80 has been designed to be both efficient and easily adaptable to a variety of applications. The system is implemented entirely in Prolog , a programming language based on logic . With the aid of a logic-based grammar formalism called extraposition grammars , Chat-80 translates English questions into the Prolog subset of logic . The resulting logical expression is then transformed by a planning algorithm into efficient Prolog , cf. query optimisation in a relational database . Finally, the Prolog form is executed to yield the answer. 