New material on formation sequences for first-order logic and propositional logic

[beastaugh]

New material on formation sequences for first-order logic and propositional logic, based on what can be found in Smullyan and more recent textbooks like van Dalen.

[beastaugh]

Had to fix some compile errors, hence the force push (it was working for me locally but the CI must be loading some package that does something with underscores that my local setup doesn't). Should be working ok now.

[rzach]

Will have look soon. (Underscores are active (subscripts!) so they don't work outside math.)

[rzach]

Thanks!

[rzach]

(I made a few changes fyi)

[beastaugh]

Looks good, thanks!