code: def subdivide(self, face_index=None):
          
          vertices, faces = remesh.subdivide(vertices=self.vertices,
                                             faces=self.faces,
                                             face_index=face_index)
          return Trimesh(vertices=vertices, faces=faces)
docs: Subdivide a mesh, with each subdivided face replaced with four
          smaller faces.
  
          Parameters
          ----------
          face_index: (m,) int or None
            If None all faces of mesh will be subdivided
            If (m,) int array of indices: only specified faces will be
            subdivided. Note that in this case the mesh will generally
            no longer be manifold, as the additional vertex on the midpoint
            will not be used by the adjacent faces to the faces specified,
            and an additional postprocessing step will be required to
            make resulting mesh watertight