once(Goal) :-
    call(Goal), !.

once/1 can in many cases be replaced with ->/2. The only difference is how the cut behaves (see !/0). The following two clauses below are identical. Be careful about the interaction with ;/2. The library(apply_macros) library defines an inline expansion of once/1, mapping it to (Goal->true;fail). Using the full if-then-else constructs prevents its semantics from being changed when embedded in a ;/2 disjunction.

1) a :- once((b, c)), d.
2) a :- b, c -> d.