proposition: Lemma omf_valid f h t : t \in dom (f h) -> valid h.
proof: Proof. by move/dom_valid; rewrite valid_omf. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)