docs: def _identifyBranches(self):
          """ 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.
          """
  
          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))