refactor(06): use P, Q, R, for propositional variables and A, B, C for formulas
refactor(06): use P, Q, R, for propositional variables and A, B, C for formulas

soonhokong/logic_and_proof,soonhokong/logic_and_proof,soonhokong/logic_and_proof,fpvandoorn/logic_and_proof,fpvandoorn/logic_and_proof,fpvandoorn/logic_and_proof,leanprover/logic_and_proof,avigad/logic_and_proof,leanprover/logic_and_proof,rlewis1988/logic_and_proof,rlewis1988/logic_and_proof,rlewis1988/logic_and_proof,leanprover/logic_and_proof,avigad/logic_and_proof,avigad/logic_and_proof