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))
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.