Completes 08e6491 in reusing `maybe_visit`

:sweat: I don't know why the tests did not fail, but to keep
the same syntax as before, `collector =` is required.

Maybe `visit` changes `collector` in-place, so the result is the
same, but since I'm not sure about the side effects, I think this
PR is needed to. Sorry! :sweat:
Merge pull request #326 from claudiob/reuse-maybe-visit

Completes 08e6491 in reusing `maybe_visit`