proposition: Lemma surjective_pr f y: surjection f -> inc y (target f) -> exists2 x, inc x (source f) & related (graph f) x y .
proof: Proof. move => [ff h] ytf; move: (h _ ytf) => [x xsf ->]; ex_tac. rewrite /related; Wtac. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)