:- module bug480b.
:- interface.

:- import_module bool.
:- import_module maybe.

:- type mystate
    --->    mystate(
                visited :: bool,
                counter :: maybe(int)
            ).

:- pred visit(mystate::in, mystate::out) is det.

:- implementation.

:- import_module int.

visit(!State) :-
    !State ^ visited := yes,
    (
        !.State ^ counter = yes(N),
        !State ^ counter := yes(N + 1)
    ;
        !.State ^ counter = no
    ).