proposition: Lemma In_rangeR f1 f2 x : valid (f1 \+ f2) -> x \In range f2 -> x \In range (f1 \+ f2).
proof: Proof. by move=>W; rewrite !In_rangeX; case=>k H; exists k; apply/InR. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)