docs: def crop(im, size):
      """
      Crops an image in the center.
  
      Parameters
      ----------
      size : tuple, (height, width)
          Finally size after cropping.
      """
      diff = [im.shape[index] - size[index] for index in (0, 1)]
      im2 = im[diff[0]//2:diff[0]//2 + size[0], diff[1]//2:diff[1]//2 + size[1]]
      return im2