Disjunction: Succeeds if P succeeds or Q succeeds.
This is normally regarded as part of the
syntax of the language, but it is like a built-in predicate in that
you can say
; Q). The character
(vertical bar) can be used as an alternative to
;/2 has a distinct behaviour if the first argument
->/2 term. See
;/2 - if-then-else.