code: def _identifyBranches(self):
          
  
          if self.debug:
              sys.stdout.write("Identifying branches: ")
              start = time.clock()
  
          seen = set()
          self.branches = set()
  
          # Find all of the branching nodes in the tree, degree > 1
          # That is, they appear in more than one edge
          for e1, e2 in self.edges:
              if e1 not in seen:
                  seen.add(e1)
              else:
                  self.branches.add(e1)
  
              if e2 not in seen:
                  seen.add(e2)
              else:
                  self.branches.add(e2)
  
          if self.debug:
              end = time.clock()
              sys.stdout.write("%f s\n" % (end - start))
docs: A helper function for determining all of the branches in the
              tree. This should be called after the tree has been fully
              constructed and its nodes and edges are populated.