==> ((print "foo"))

"foo"
YES.

==> (and (= ?x 3) ((* ?x ?x) ?y))
; Note that "?y" is unified with 9
?X = 3
?Y = 9

==> ((* 3 3) 10)
NO.

==> ((floor 3 4) ?x ?y)

?X = 0
?Y = 3
==> ((floor 3 4) ?x)

?X = 0

==> ((* 3 4) ?x ?y)

?X = 12
?Y = ?0
; note that system generated variables look like:
; ?<integer>
==> ((typep 3 'integer) ?x)

?X = T

==> ((typep 3 'integer) t)

YES.

==> (and ((floor 5 3) ?x) ((floor 4 3) ?x))

?X = 1

==> ((cons 3 4) (?x . ?y))

?X = 3
?Y = 4

==> (and (= ?op *) ((list ?op 3 4) ?y) (call (?y ?z)))

?OP = *
?Y = (* 3 4)
?Z = 12

==> (and (defrel fact
 ((fact 0 1))
 ((fact ?x ?y)
 (( ?x 1) ?w)
 (fact ?w ?z)
 ((* ?z ?x) ?y)))
 (fact 10 ?result))

?X = ?0
?Y = ?1
?W = ?2
?Z = ?3
?RESULT = 3628800
KnowledgeWorks and Prolog User Guide (Unix version)  24 Mar 2017