proposition: Lemma dvdnP d m : reflect (exists k, m = k * d) (d %| m).
proof: Proof. apply: (iffP eqP) => [md0 | [k ->]]; last by rewrite modnMl. by exists (m %/ d); rewrite [LHS](divn_eq m d) md0 addn0. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq)