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
, 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
with the first clause. If
is a rottweiler the
then prevents the backward chainer from using the second clause which says "everything is nice". The fail ensures that