proposition: Lemma InL x f1 f2 : valid (f1 \+ f2) -> x \In f1 -> x \In (f1 \+ f2).
proof: Proof. by move=>W /In_find E; apply/In_find; rewrite findUnL // (find_some E). Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)