static <A> Matrix<Matrix<A>> matrixToMNGrid(Matrix<A> m, int cols, int rows) {
  if (m == null) null;
  
  // make reasonable
  cols = clamp(cols, 1, m.getWidth());
  rows = clamp(rows, 1, m.getHeight());

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