Control predicates

X ; Y

X or Y. Try X first; if it fails (possibly after being backtracked into), try Y.

(X -> Y)

If X, then try Y, otherwise fail. Y will not be backtracked into.

(X -> Y ; Z)

If X, then try Y, else try Z. X will not be backtracked into.

not X

(Sometimes written \+X or not(X)) Succeed only when X fails.

true

Succeed once, but fail when backtracked into.

repeat

Always succeed, even when backtracked into.

fail

Never succeed.

!

(Pronounced ``cut".) Acts like true, but cannot be backtracked past, and prevents any other clauses of the predicate it occurs in from being tried.

abort

Return immediately to the top-level Prolog prompt.