The Chat-80 natural-language query system, which I developed with David H. D. Warren for my dissertation work between 1979 and 1982, is sometimes still sought by people interested in natural-language processing in Prolog. Here is a version of Chat-80 that runs under Quintus Prolog, as a gzip (GNU zip) tar archive. Chat-80 and the main ideas underlying it are described in [Pereira and Warren, 1980], [Pereira, 1981] and [Warren and Pereira, 1982]. A lot more detail can be found in my dissertation, which may still be available as Technical Note 275 from SRI International's Artificial Intelligence Center. For various technical and copyright reasons, the publications above are unfortunately not available online.