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:- module flip_impl.

:- interface.

:- pred flipflip(tree(T), tree(T)).
:- mode flipflip(in, out) is det.

:- type tree(T)
    --->    leaf(T)
    ;       tree(tree(T), T, tree(T)).

:- type nat
    --->    zero
    ;       s(nat).

:- implementation.

flipflip(XT, YT) :-
    flip(XT, TT),
    flip(TT, YT).

:- pred flip(tree(T), tree(T)).
:- mode flip(in, out) is det.

flip(leaf(X), leaf(X)).
flip(tree(XT, Info, YT), tree(FYT, Info, FXT)) :-
    flip(XT, FXT),
    flip(YT, FYT).