Convenience

These predicates are standard and often self-explanatory, so they are described only briefly.

true

Always succeeds.

otherwise

Same as true/0.

fail

Always fails.

X = Y

Defined as if by the clause ``Z=Z'', i.e. X and Y are unified.

Succeeds if X and Y are not unifiable, fails if X and Y are unifiable. It is thus equivalent to \+(X = Y).