A prover that is able to refute HOL sequents/formulas (or subsets of HOL, like propositional logic).
A prover that is able to refute HOL sequents/formulas (or subsets of HOL, like propositional logic).
TODO: exceptions to indicate that a formula is not supported (e.g. for propositional provers).
Implementors may want to override isValid(seq) to avoid parsing proofs.