The classical CHAT80 natural language system
The CHAT80 system has been developed in the 70s and 80s by Fernando C.N. Pereira and David H.D. Warren. It implements a natural language question answering system that answers questions about the world: countries, cities, rivers, etc. It does so by parsing the question, translate the parse to a Prolog query and run this against its database.
This version is derived from the original via Quintus Prolog after some compatibility modifications for SWI-Prolog and adding a module header that allows using it safely together with other applications.
The code is definitely dated. Still, it provides a nice example using Prolog for parsing, assigning meaning and querying.
The copyright is as far as we know with the original authors and made
available under a classical academic use license. See LICENSE in the
prolog/chat80 directory. The content of that file was copied from the
Python NLTK data package
that includes the chat80 files.