Definition col_mxAx := col_mxA.

Lemma col'Kr m n1 n2 j2 (A1 : 'M_(m, n1)) (A2 : 'M_(m, n2)) : col' (rshift n1 j2) (@row_mx m n1 n2 A1 A2) = castmx (erefl m, mx'_cast n1 j2) (row_mx A1 (col' j2 A2)).