proposition: Lemma bijective_pr f y: bijection f -> inc y (target f) -> exists! x, (inc x (source f) /\ Vf f x = y).
proof: Proof. move=> [[_ inj] [_ sf]] yt; apply /unique_existence; split. by move: (sf _ yt)=> [x p1 p2]; exists x. by move=> x x' [xs Wx] [x's Wx']; rewrite -Wx' in Wx; apply: inj. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)