proposition: Lemma codom_apply {A B : Type} {f : A -> B} {x : A} : f x \in codom f.
proof: Proof. eexists; eauto. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)