I am looking for a proof checker for modal logic using natural deduction or sequent calculus.
I am trying to learn Isabelle, but I think there should be a simpler solution.
Although I can rely on mentally checking proofs I realize it is very easy to make mistakes and I do not trust my ability to manually check even simple proofs.
I will list here some possibilities I am exploring:
Ross Kirsling's Modal Logic Playground
Another useful resource is Frederic Fitch's Symbolic Logic: An Introduction. He lists the natural deduction rules for various modal logics. David F. Siemens, Jr, "Fitch-Style Rules for Many Modal Logics" Notre Dame Journal of Formal Logic Volume XVIII, Number 4, October 1977 summarize these. These are not proof checkers, but provide what one should expect to see in a proof checker.