code: def vertex_neighbors(self):
          
          graph = self.vertex_adjacency_graph
          neighbors = [list(graph.neighbors(i)) for
                       i in range(len(self.vertices))]
          return np.array(neighbors)
docs: The vertex neighbors of each vertex of the mesh, determined from
          the cached vertex_adjacency_graph, if already existent.
  
          Returns
          ----------
          vertex_neighbors : (len(self.vertices),) int
            Represents immediate neighbors of each vertex along
            the edge of a triangle
  
          Examples
          ----------
          This is useful for getting nearby vertices for a given vertex,
          potentially for some simple smoothing techniques.
  
          >>> mesh = trimesh.primitives.Box()
          >>> mesh.vertex_neighbors[0]
          [1,2,3,4]