code: def iter_edges(self, cached_content=None):
          
          if not cached_content:
              cached_content = self.get_cached_content()
          all_leaves = cached_content[self]
          for n, side1 in six.iteritems(cached_content):
              yield (side1, all_leaves - side1)
docs: Iterate over the list of edges of a tree. Each egde is represented as a
          tuple of two elements, each containing the list of nodes separated by
          the edge.