Section OneMatrix.
Variables (m n : nat) (A : 'M[aT]_(m, n)).

Lemma col_mx_eq0 (m1 m2 n : nat) (A1 : 'M_(m1, n)) (A2 : 'M_(m2, n)):
(col_mx A1 A2 == 0) = (A1 == 0) && (A2 == 0).