:- module maybe.
:- interface.
:- type maybe_error.
:- type maybe(T).
:- type maybe_error(T).
:- type maybe_error(T, E).
:- inst (maybe(I)) == bound(no ; yes(I)).
:- inst (maybe_error(I)) == bound(error(ground) ; ok(I)).
:- inst (maybe_error_ok(I)) == bound(ok(I)).
:- inst (maybe_yes(I)) == bound(yes(I)).
