| Name | Expression | Return Type | Description | 
| Begin Traversal | vis.begin_traversal() | void | This is invoked once per traversal, before the traversal begins. | 
| Begin Face | vis.begin_face() | void | This is invoked once for each face, before any vertices or edges on the face
are visited. | 
| Next Vertex | vis.next_vertex(v) | void | This is invoked when a vertex is encountered while traversing a face. | 
| Next Edge | vis.next_edge(e) | void | This is invoked when an edge is encountered while traversing a face. | 
| End Face | vis.end_face() | void | This is invoked once for each face, after all vertices and edges on the face
are visited. | 
| End Traversal | vis.end_traversal() | void | This is invoked once per traversal, after the traversal ends. |