iptor
</tt></td>
<td>
The type for the objects used to identify edges in the graph.
</td>
</tr>

<tr>
<td><tt>
directed_category
</tt></td>
<td>
This says whether the graph is undirected (<tt>undirected_tag</tt>)
or directed (<tt>directed_tag</tt>).
</td>
</tr>

<tr>
<td><tt>
edge_parallel_category
</tt></td>
<td>
This says whether the graph allows parallel edges to be inserted
(<tt>allow_parallel_edge_tag</tt>) or if it automatically removes
parallel edges (<tt>disallow_parallel_edge_tag</tt>).
</td>
</tr>