A.13.1 The defdetpred form

The syntax of this form is

which defines a simple predicate that just runs lisp code and doesn't have to unify any variables. Arguments are referenced with: (special-arg <argnum> ) . The body succeeds by default, but if a failure case arises, use: (detpred-fail <name> <num args> ) .

For example

(defdetpred my-integer 1

  (unless (integerp (special-arg 0))

  (detpred-fail my-integer 1)))

KnowledgeWorks and Prolog User Guide (Unix version) - 6 Dec 2011