code: def face_adjacency_unshared(mesh):
      
  
      # the non- shared vertex index is the same shape as face_adjacnecy
      # just holding vertex indices rather than face indices
      vid_unshared = np.zeros_like(mesh.face_adjacency,
                                   dtype=np.int64)
      # loop through both columns of face adjacency
      for i, adjacency in enumerate(mesh.face_adjacency.T):
          # faces from the current column of face adjacency
          faces = mesh.faces[adjacency]
          shared = np.logical_or(
              faces == mesh.face_adjacency_edges[:, 0].reshape((-1, 1)),
              faces == mesh.face_adjacency_edges[:, 1].reshape((-1, 1)))
          vid_unshared[:, i] = faces[np.logical_not(shared)]
      return vid_unshared
docs: Return the vertex index of the two vertices not in the shared
      edge between two adjacent faces
  
      Parameters
      ----------
      mesh : Trimesh object
  
      Returns
      -----------
      vid_unshared : (len(mesh.face_adjacency), 2) int
          Indexes of mesh.vertices