proposition: Lemma p2graph_target x y : inc (J x y) (graph f) -> inc y (target f).
proof: Proof. move=> Jgf; rewrite (Vf_pr Jgf); apply: Vf_target; apply: (p1graph_source Jgf). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)