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:- module fold.

:- interface.

:- type list(T)
    --->    []
    ;       [T | list(T)].

:- type sym
    --->    a
    ;       b
    ;       c
    ;       d.

:- pred fold(sym, list(sym), sym).
:- mode fold(in, in, out) is semidet.

:- implementation.

fold(X, [Y | Ys], Z) :-
    xop(X, Y, V),
    fold(V, Ys, Z).
fold(X, [], X).

:- pred xop(sym, sym, sym).
:- mode xop(in, in, out).

xop(a, b, c).