proposition: Lemma fsfun_with f x y : [fsfun f with x |-> y] x = y.
proof: Proof. by rewrite fsfun_withE eqxx. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrnat eqtype ssrfun seq) mathcomp(choice path finset finfun fintype bigop)