proposition: Lemma sqrnD_sub' m n: n <= m -> (m + n) ^ 2 = 4 * (m * n) + (m - n) ^ 2.
proof: Proof. move => h; rewrite sqrnD sqrnB // -[4]/(2+2) mulnDl - [in RHS]addnA addnC. by rewrite subnKC // nat_Cauchy. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq path fintype div bigop binomial) mathcomp(prime finset ssralg ssrnum ssrint)