proposition: Lemma pfree_vars_closed_pr X x: pclosed x -> (pformula_valid_on X x <-> inc (Q x) (formula_val X (P x))).
proof: Proof. by move => /(pfree_vars_closed_pr1 X) [ _]. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)