code: def cosinebell(n, fraction):
      
  
      mask = np.ones(n)
      nmasked = int(fraction * n)
      for i in range(nmasked):
          yval = 0.5 * (1 - np.cos(np.pi * float(i) / float(nmasked)))
          mask[i] = yval
          mask[n - i - 1] = yval
  
      return mask
docs: Return a cosine bell spanning n pixels, masking a fraction of pixels
  
      Parameters
      ----------
      n : int
          Number of pixels.
      fraction : float
          Length fraction over which the data will be masked.