s(CASP)

s(CASP) is an open source goal-directed constraint answer set programming language. It features natural language explanations for conclusions, hypothetical reasoning, and constraints.

It was originally implemented in the Ciao programming language. Work is underway on a re-implementation in SWI-Prolog.

A blog post explaining the details of the differences between s(CASP) and other logical programming languages is available here.