current_predicate(X) attempts to unify X against all currently defined predicates.
Examples
?- current_predicate(!/0).
yes;
no
?- current_predicate(!/X).
X = 0
yes;
no
?- current_predicate(!/1).
no
?- current_predicate(doesnt_exist/1).
no
?- current_predicate(call/X).
X = 1
yes;
X = 2
yes;
X = 3
yes;
X = 4
yes;
X = 5
yes;
X = 6
yes;
X = 7
yes;
X = 8
yes;
X = 9
yes;
X = 10
yes;
no