proposition: Lemma In_rangeUn f1 f2 x : x \In range (f1 \+ f2) -> x \In range f1 \/ x \In range f2.
proof: Proof. by rewrite !In_rangeX; case=>k /InUn [] H; [left | right]; exists k. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)