code: public void removeEdge(N a, N b)
      {
          if(!containsBoth(a, b))
              return;
          nodes.get(a).getOutgoing().remove(b);
          nodes.get(b).getIncoming().remove(a);
      }
docs: Removes a directed edge from the network connecting <tt>a</tt> to <tt>b</tt>.
  If <tt>a</tt> and <tt>b</tt> are not nodes in the graph, nothing occurs.
  @param a the parent node
  @param b the child node