code: def face_subset(self, face_index):
          
          if self.defined:
              result = ColorVisuals(
                  face_colors=self.face_colors[face_index])
          else:
              result = ColorVisuals()
  
          return result
docs: Given a mask of face indices, return a sliced version.
  
          Parameters
          ----------
          face_index: (n,) int, mask for faces
                      (n,) bool, mask for faces
  
          Returns
          ----------
          visual: ColorVisuals object containing a subset of faces.