proposition: Lemma get_column x y g : size g = hw * hw -> x < hw -> y < hw -> get (x, y) g = nth out (column y g) x.
proof: Proof. move=> Hs xLhw yLhw. rewrite /get /column /pos2n nth_take_and_drop ?muln1 //=. - by rewrite nth_drop divn1 modn1 addn0 addnC. - by apply: leq_ltn_trans xLhw. rewrite size_drop addn1 Hs; nia. Qed.
imports: mathcomp(all_ssreflect) mathcomp(zify)