title: subtree with networkX
body: <p>In networkX, I have a tree as DiGraph().</p>
  <pre><code>#!/usr/bin/python
  # -*- coding: utf-8 -*-
  import networkx as nx
  t = nx.DiGraph()
  t.add_edge(1,'r')
  t.add_edge(2,'r')
  t.add_edge(3,'r')
  t.add_edge(4,2)
  t.add_edge(5,2)
  t.add_edge(6,5)
  print t.edges() 
  </code></pre>
  <p>If a take the node 2 of tree.<br />
  how I can get the subtree of 2 ?</p>
  <h3>Edit</h3>
  <p>I expected this subtree</p>
  <pre><code>[(4,2),(5,2),(6,5)]
  </code></pre>