code: def _build_kth_tree(self):
          
          anchor = self.get_anchor()
          aux_sorted = self._sort_tau_by_y(anchor)
          edges = self.previous_tree.edges
  
          for itr in range(self.n_nodes - 1):
              right = int(aux_sorted[itr, 0])
              left_parent, right_parent = Edge.sort_edge([edges[anchor], edges[right]])
              new_edge = Edge.get_child_edge(itr, left_parent, right_parent)
              new_edge.tau = aux_sorted[itr, 1]
              self.edges.append(new_edge)
docs: Build k-th level tree.