In order to call a function on a numpy array, we have to index the array properly.

Given the array `arr = np.ndarray(64).reshape((8, 8))`, we can obtain a submatrix `sub = arr[::2, ::2]`
In order to perform an operation on (x, y) in sub, we have to convert it into an operation on arr.
In general, operations need to be performed on `ndarray.base`.