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:- module dds3_13.

:- interface.

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

:- pred duplicate(list(T)::in, list(T)::out) is det.

:- implementation.

duplicate([], []).
duplicate([X | Y], [X, X | Z]) :-
    duplicate(Y, Z).