proposition: Lemma BRprod_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: (RSp br dr)(RSp cr dr)=> sa sb. by rewrite -!BRprodA // (BRprod_CA br cr dr). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)