static Matrix<Matrix<Int>> matrixToMNGrid(Matrix<A> m, int cols, int rows) {
  if (m == null) null;

  LL<Rect> rects = gridOfRects3(m.getWidth(), m.getHeight(), cols, rows);
  LL<Matrix<Int>> matrices = mapLevelTwo(r -> new SubMatrix(m, r), rects);
  ret listOfListsToMatrix(matrices);
}