proposition: Lemma expRB (n m : nat) r : (m <= n)%nat -> r <> 0 -> r ^ (n - m) = r ^ n / (r ^ m).
proof: Proof. move=> Hr ab. by rewrite (pow_RN_plus r _ m) // plusE -minusE subnK // powRV //; exact/eqP. Qed.
imports: mathcomp(all_ssreflect ssralg ssrnum) Reals mathcomp(lra) mathcomp(Rstruct)