The once(X) goal succeeds if an attempt to satisfy the goal represented by the term X succeeds. No attempt is made to retry the goal during backtracking - it is only evaluated once.
Examples
?- once(repeat).
yes
?- once(true).
yes
?- once((true,true,true)).
yes
?- once((true;true;true)).
yes
?- once((fail;true;true)).
yes
?- once((true;fail;true)).
yes
?- once((true;true;fail)).
yes
?- once(fail).
no
?- once((fail;fail;fail)).
no
?- once((fail,fail,fail)).
no
?- once((true,true,fail)).
no
?- once((true,fail,true)).
no
?- once((fail,true,true)).
no