code: def set_fixed_mask(self, image):
          
          if not isinstance(image, iio.ANTsImage):
              raise ValueError('image must be ANTsImage type')
  
          if image.dimension != self.dimension:
              raise ValueError('image dim (%i) does not match metric dim (%i)' % (image.dimension, self.dimension))
  
          self._metric.setFixedImage(image.pointer, True)
          self.fixed_mask = image
docs: Set Fixed ANTsImage Mask for metric