proposition: Lemma ge0_abs x : x >= 0 -> `|x| == x.
proof: Proof. move=> le0x; split=> [a ltxa | a []//]; split=> //. by exists 0%R; rewrite // oppr_lt0; apply/leRq: a ltxa. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq order) mathcomp(ssralg ssrnum ssrint rat) fourcolor(real) Coq(Setoid Morphisms)