Cleanup of @spcfox's modal logic pull request

[EgbertRijke]

@spcfox made an impressive pull request, but unfortunately we haven't heard from him since. This is an attempt to bring his pull request to a mergeable state.

[fredrik-bakke]

Very glad that someone is taking care of this! Do you want a third pair of eyes to look at this before it is merged?

[spcfox]

Hello!
I am very glad that my contribution is important to you. I would be very interested in completing this pull request, but unfortunately for personal reasons I do not have the energy to do so right now.

[EgbertRijke]

Hi @spcfox!

I'm glad to hear from you and I hope you are doing well. You have made a heroic effort with your pull request, and indeed I am very happy to take on the task towards merging it. I completely understand that a personal situation might be getting in the way, and you don't need to worry about the pull request if you don't have the energy for it. We will make sure that you will be appropriately attributed for the work that you have done.

Take care, and I hope to hear from you again sometime!