proposition: Lemma open x a : x < a -> exists2 b, x < b & (b < a)%R.
proof: Proof. by case: x a => x []. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq order) mathcomp(ssralg ssrnum ssrint rat) fourcolor(real) Coq(Setoid Morphisms)