proposition: Lemma sqredD x y : x + y \is a fin_num -> (x + y) ^+ 2 = x ^+ 2 + x * y *+ 2 + y ^+ 2.
proof: Proof. case: x y => [x||] [y||] // _. by rewrite -EFinM -EFin_dnatmul -!EFin_expe -!dEFinD sqrrD. Qed.
imports: HB(structures) mathcomp(all_ssreflect all_algebra finmap) mathcomp(mathcomp_extra) signed