Section OneMatrix.
Variables (m n : nat) (A : 'M[aT]_(m, n)).
Definition nz_row m n (A : 'M_(m, n)) :=
oapp (fun i => row i A) 0 [pick i | row i A != 0].

Lemma nz_row_eq0 m n (A : 'M_(m, n)) : (nz_row A == 0) = (A == 0).