def _DX(X):
    """Computes the X finite derivarite along y and x.

    Arguments
    ---------
    X: (m, n, l) numpy array
        The data to derivate.

    Returns
    -------
    tuple
        Tuple of length 2 (Dy(X), Dx(X)).

    Note
    ----
    DX[0] which is derivate along y has shape (m-1, n, l).
    DX[1] which is derivate along x has shape (m, n-1, l).
    """
    return (X[1:, :, :] - X[:-1, :, :],         # D along y
            X[:, 1:, :] - X[:, 0:-1, :])        # D along x