code: def erosion(x, radius=3):
      
      mask = disk(radius)
      x = _erosion(x, selem=mask)
      return x
docs: 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.