original_docstring: 
          This is invoked on the back edges in the graph.
          For an undirected graph there is some ambiguity between tree edges
          and back edges since the edge :math:`(u, v)` and :math:`(v, u)` are the
          same edge, but both the `tree_edge()` and `back_edge()` functions will be
          invoked. One way to resolve this ambiguity is to record the tree edges,
          and then disregard the back-edges that are already marked as tree edges.
          An easy way to record tree edges is to record predecessors at the
          `tree_edge` event point.
          
docstring: This is invoked on the back edges in the graph.
docstring_tokens: This is invoked on the back edges in the graph .
code: def back_edge(self, e):
          """
          This is invoked on the back edges in the graph.
          For an undirected graph there is some ambiguity between tree edges
          and back edges since the edge :math:`(u, v)` and :math:`(v, u)` are the
          same edge, but both the `tree_edge()` and `back_edge()` functions will be
          invoked. One way to resolve this ambiguity is to record the tree edges,
          and then disregard the back-edges that are already marked as tree edges.
          An easy way to record tree edges is to record predecessors at the
          `tree_edge` event point.
          """
          return
code_tokens: def back_edge ( self , e ) : return
short_docstring: This is invoked on the back edges in the graph.
short_docstring_tokens: This is invoked on the back edges in the graph .
comment: """
          This is invoked on the back edges in the graph.
          For an undirected graph there is some ambiguity between tree edges
          and back edges since the edge :math:`(u, v)` and :math:`(v, u)` are the
          same edge, but both the `tree_edge()` and `back_edge()` functions will be
          invoked. One way to resolve this ambiguity is to record the tree edges,
          and then disregard the back-edges that are already marked as tree edges.
          An easy way to record tree edges is to record predecessors at the
          `tree_edge` event point.
          """