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.

X $\backslash$= Y

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