proposition: Lemma gerB_real x y : x <= y -> x - y \is real.
proof: Proof. by move=> le_xy; rewrite ler0_real // subr_le0. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq choice) mathcomp(ssrAC div fintype path bigop order finset fingroup) mathcomp(ssralg poly)