proposition: Lemma BRsum_ACA a b c d: realp a -> realp b -> realp c -> realp d -> (a +r b) +r (c +r d) = (a +r c) +r (b +r d).
proof: Proof. move => ar br cr dr. move: (RSs br dr)(RSs cr dr)=> sa sb. by rewrite -!BRsumA // (BRsum_CA br cr dr). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)