proposition: Lemma surjective_pr1 f: function f -> Imf f = target f -> surjection f.
proof: Proof. move=> ff ift; split => // y; rewrite -ift. move/dirim_P=> [x pa pb];ex_tac; Wtac. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)