[m-rev.] for post-commit review: fix mantis bug 480 fully

[Peter Wang]

On Wed, 07 Aug 2019 14:59:20 +0200 (CEST), "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> For post-commit review by Peter.
> 

That looks fine, thanks.

I have a new test case for you.

Peter
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:- 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
    ).