proposition: Lemma imsetT T U (f : T -> U) : f @: setT <--> codom f.
proof: Proof. move=> y; split; first by case=> x [_ fx]; exists x. by case=> x fx; exists x. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)