ref: baea4aa939861fd4efbc71b96f93ba890f01ac40
dir: /stdlib.pl/
% Logic and control predicates \+ Goal :- call(Goal), !, fail. \+ Goal. once(Goal) :- call(Goal), !. repeat :- true ; repeat. % Control structures. true. If -> Then :- If, !, Then. If -> Then ; _ :- If, !, Then. _ -> _ ; Else :- !, Else. If ; _ :- If. _ ; Else :- Else. % Term unification A = A. A \= B :- \+ A = B.