proposition: Lemma mem_rangeX v f : v \in range f <-> exists k, (k, v) \In f.
proof: Proof. by split; [move/mem_seqP/In_rangeX|move/In_rangeX/mem_seqP]. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)