proposition: Lemma imsetU1 a (A : {set aT}) : f @: (a |: A) = f a |: (f @: A).
proof: Proof. by rewrite imsetU imset_set1. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)