proposition: Lemma sqrrD1 x : (x + 1) ^+ 2 = x ^+ 2 + x *+ 2 + 1.
proof: Proof. rewrite exprD1n !big_ord_recr big_ord0 /= add0r. by rewrite addrC addrA addrAC. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop prime binomial)