proposition: Lemma injective_pr f y: injection f -> singl_val (fun x => related (graph f) x y).
proof: Proof. rewrite /related;move=> [ff injf] x x' r1 r2; apply: injf. - by Wtac. - by Wtac. - by rewrite - (Vf_pr ff r1) (Vf_pr ff r2). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)