?- (between(1,2,X) ; X = a).
X = 1 ;
X = 2 ;
X = a.

It is strongly advised to always use parenthesis around disjunctions. Conjunctions inside a disjunction should not use parenthesis. Traditionally the ; is placed at the start of the line rather than at the end because a ; at the end of a line is easily overlooked. Below is an example of the preferred style used in SWI-Prolog.^{75Some users prefer a newline after the ;.}

p :-
    a,
    (   b,
        c
    ;   d
    ).

Although ;/2 is a control structure that is normally handled by the compiler, SWI-Prolog implements ;/2 as a true predicate to support call/2 and friends as well as to allow for querying predicate properties, for example to support code analysis.