feat(nd_for_prop_logic): more exercises
feat(nd_for_prop_logic): more exercises

rlewis1988/logic_and_proof,avigad/logic_and_proof,avigad/logic_and_proof,rlewis1988/logic_and_proof,leanprover/logic_and_proof,leanprover/logic_and_proof,avigad/logic_and_proof,leanprover/logic_and_proof,rlewis1988/logic_and_proof