proposition: Lemma Rminus_lt_0 : forall a b, a < b <-> 0 < b - a.
proof: Proof. Proof. intros. pattern a at 1. rewrite <- (Rplus_0_l a). split ; apply Rlt_minus_r. Qed.
imports: Coq(Reals Psatz ssreflect) Coq(ssrbool) mathcomp(seq)