docs: def keep_segments(self, segments_to_keep, preserve_segmentation=True):
          '''
          Keep the faces and vertices for given segments, discarding all others.
          When preserve_segmentation is false self.segm is discarded for speed.
          '''
          v_ind, f_ind = self.vertex_indices_in_segments(segments_to_keep, ret_face_indices=True)
          self.segm = {name: self.segm[name] for name in segments_to_keep}
  
          if not preserve_segmentation:
              self.segm = None
              self.f = self.f[f_ind]
              if self.ft is not None:
                  self.ft = self.ft[f_ind]
  
          self.keep_vertices(v_ind)