proposition: Lemma setIb_constant f x: constantgp f -> inc x (domain f) -> intersectionb f = Vg f x.
proof: Proof. move => p1 p2. have nef: nonempty f by apply /domain_set0P; ex_tac. set_extens t; first by move /(setIb_P nef); apply. by move => h; apply /(setIb_P nef) => i idf;rewrite (proj2 p1 _ _ idf p2). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)