def _build_first_tree(self):
        """Build first level tree."""
        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)
Build first level tree.