proposition: Lemma fgraph_pr f x y y': fgraph f -> inc (J x y) f -> inc (J x y') f -> y = y'.
proof: Proof. move=> [_ fg] ia ib. move: (fg _ _ ia ib); aw => h; exact: (pr2_def (h (erefl x))). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)