code: def merge(self, img):
          
          raise NotImplementedError("This method has not be implemented for "
                                    "xarray support.")
          if self.is_empty():
              raise ValueError("Cannot merge an empty image.")
  
          if self.mode != img.mode:
              raise ValueError("Cannot merge image of different modes.")
  
          selfmask = self.channels[0].mask
          for chn in self.channels[1:]:
              selfmask = np.ma.mask_or(selfmask, chn.mask)
  
          for i in range(len(self.channels)):
              self.channels[i] = np.ma.where(selfmask,
                                             img.channels[i],
                                             self.channels[i])
              self.channels[i].mask = np.logical_and(selfmask,
                                                     img.channels[i].mask)
docs: Use the provided image as background for the current *img* image,
          that is if the current image has missing data.