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:- module dds3_17.

:- interface.

:- type expr
    --->    true
    ;       false
    ;       or(expr, expr)
    ;       and(expr, expr).

:- pred dis(expr::in) is semidet.
:- pred con(expr::in) is semidet.

:- implementation.

dis(or(B1, B2)) :-
    con(B1),
    dis(B2).
dis(B) :-
    con(B).

con(and(B1, B2)) :-
    dis(B1),
    con(B2).
con(B) :-
    bool(B).

:- pred bool(expr).
:- mode bool(in).

bool(true).
bool(false).