def erosion(x, radius=<NUM_LIT:3>):
mask = disk(radius)<EOL>x = _erosion(x, selem=mask)<EOL>return x<EOL>
Return greyscale morphological erosion of an image,
    see `skimage.morphology.erosion <http://scikit-image.org/docs/dev/api/skimage.morphology.html#skimage.morphology.erosion>`__.

    Parameters
    -----------
    x : 2D array
        A greyscale image.
    radius : int
        For the radius of mask.

    Returns
    -------
    numpy.array
        A processed greyscale image.