code: def _build_first_tree(self):
          
          tau_sorted = self._sort_tau_by_y(0)
          for itr in range(self.n_nodes - 1):
              ind = int(tau_sorted[itr, 0])
              name, theta = Bivariate.select_copula(self.u_matrix[:, (0, ind)])
  
              new_edge = Edge(itr, 0, ind, name, theta)
              new_edge.tau = self.tau_matrix[0, ind]
              self.edges.append(new_edge)
docs: Build first level tree.