proposition: Lemma uvalidL_fin x y : uvalid (ujoin x y) -> uvalid x.
proof: Proof. move/forallP=>H; apply/forallP=>t; move: (H t). by rewrite sel_fin=>/validL. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq bigop fintype finset) pcm(options axioms prelude seqperm pred seqext)