proposition: Lemma range_valid k f : k \in range f -> valid f.
proof: Proof. by move/mem_seqP/In_range_valid. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path bigop) pcm(options prelude finmap seqperm pred seqext) pcm(pcm morphism)