cut is a standard prolog predicate. When first called it succeeds and freezes certain choices made by the backward chainer up to this point. It may no longer attempt to resatisfy any of the goals between the start of clause and the
cut, and it may not attempt to use any other clauses to satisfy the same goal.
implements "everything is nice unless it is a rottweiler". First the backward chainer will attempt to prove
(nice fido) with the first clause. If
fido is a rottweiler the
cut then prevents the backward chainer from using the second clause which says "everything is nice". The fail ensures that
(nice fido) fails.
KnowledgeWorks and Prolog User Guide (Unix version) - 24 Mar 2017