How to use the thewalrus._hafnian.hafnian function in thewalrus

raise ValueError("The covariance matrix does not correspond to a pure state")

    rpt = i
    beta = Beta(mu, hbar=hbar)
    Q = Qmat(cov, hbar=hbar)
    A = Amat(cov, hbar=hbar)
    (n, _) = cov.shape
    N = n // 2
    B = A[0:N, 0:N].conj()
    alpha = beta[0:N]

    if np.linalg.norm(alpha) < tol:
        # no displacement
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            B_rpt = reduction(B, rpt)
            haf = hafnian(B_rpt)
        else:
            haf = hafnian_repeated(B, rpt)
    else:
        gamma = alpha - B @ np.conj(alpha)
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            B_rpt = reduction(B, rpt)
            np.fill_diagonal(B_rpt, reduction(gamma, rpt))
            haf = hafnian(B_rpt, loop=True)
        else:
            haf = hafnian_repeated(B, rpt, mu=gamma, loop=True)

    if include_prefactor:
        pref = np.exp(-0.5 * (np.linalg.norm(alpha) ** 2 - alpha @ B @ alpha))
        haf *= pref

    return haf / np.sqrt(np.prod(fac(rpt)) * np.sqrt(np.linalg.det(Q)))

probs1 = np.zeros([cutoff + 1], dtype=np.float64)
        kk = np.arange(k + 1)
        mu_red, V_red = reduced_gaussian(local_mu, cov, kk)

        if approx:
            Q = Qmat(V_red, hbar=hbar)
            A = Amat(Q, hbar=hbar, cov_is_qmat=True)

        for i in range(cutoff):
            indices = result + [i]
            ind2 = indices + indices
            if approx:
                factpref = np.prod(fac(indices))
                mat = reduction(A, ind2)
                probs1[i] = (
                    hafnian(np.abs(mat.real), approx=True, num_samples=approx_samples) / factpref
                )
            else:
                probs1[i] = density_matrix_element(
                    mu_red, V_red, indices, indices, include_prefactor=True, hbar=hbar
                ).real

        if approx:
            probs1 = probs1 / np.sqrt(np.linalg.det(Q).real)

        probs2 = probs1 / prev_prob
        probs3 = np.maximum(
            probs2, np.zeros_like(probs2)
        )  # pylint: disable=assignment-from-no-return
        ssum = np.sum(probs3)
        if ssum < 1.0:
            probs3[-1] = 1.0 - ssum

B = A[0:N, 0:N].conj()
    alpha = beta[0:N]

    if np.linalg.norm(alpha) < tol:
        # no displacement
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            B_rpt = reduction(B, rpt)
            haf = hafnian(B_rpt)
        else:
            haf = hafnian_repeated(B, rpt)
    else:
        gamma = alpha - B @ np.conj(alpha)
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            B_rpt = reduction(B, rpt)
            np.fill_diagonal(B_rpt, reduction(gamma, rpt))
            haf = hafnian(B_rpt, loop=True)
        else:
            haf = hafnian_repeated(B, rpt, mu=gamma, loop=True)

    if include_prefactor:
        pref = np.exp(-0.5 * (np.linalg.norm(alpha) ** 2 - alpha @ B @ alpha))
        haf *= pref

    return haf / np.sqrt(np.prod(fac(rpt)) * np.sqrt(np.linalg.det(Q)))

beta = Beta(mu, hbar=hbar)
    A = Amat(cov, hbar=hbar)
    if np.linalg.norm(beta) < tol:
        # no displacement
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            A_rpt = reduction(A, rpt)
            haf = hafnian(A_rpt)
        else:
            haf = hafnian_repeated(A, rpt)
    else:
        # replace the diagonal of A with gamma
        gamma = beta.conj() - A @ beta
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            A_rpt = reduction(A, rpt)
            np.fill_diagonal(A_rpt, reduction(gamma, rpt))
            haf = hafnian(A_rpt, loop=True)
        else:
            haf = hafnian_repeated(A, rpt, mu=gamma, loop=True)

    if include_prefactor:
        haf *= prefactor(mu, cov, hbar=hbar)

    return haf / np.sqrt(np.prod(fac(rpt)))

probs1 = np.zeros([cutoff + 1], dtype=np.float64)
        kk = np.arange(k + 1)
        mu_red, V_red = reduced_gaussian(local_mu, cov, kk)

        if approx:
            Q = Qmat(V_red, hbar=hbar)
            A = Amat(Q, hbar=hbar, cov_is_qmat=True)

        for i in range(cutoff):
            indices = result + [i]
            ind2 = indices + indices
            if approx:
                factpref = np.prod(fac(indices))
                mat = reduction(A, ind2)
                probs1[i] = (
                    hafnian(np.abs(mat.real), approx=True, num_samples=approx_samples) / factpref
                )
            else:
                probs1[i] = density_matrix_element(
                    mu_red, V_red, indices, indices, include_prefactor=True, hbar=hbar
                ).real

        if approx:
            probs1 = probs1 / np.sqrt(np.linalg.det(Q).real)

        probs2 = probs1 / prev_prob
        probs3 = np.maximum(
            probs2, np.zeros_like(probs2)
        )  # pylint: disable=assignment-from-no-return
        ssum = np.sum(probs3)
        if ssum < 1.0:
            probs3[-1] = 1.0 - ssum

used to scale the result.
        tol (float): tolerance for determining if displacement is negligible
        hbar (float): the value of :math:`\hbar` in the commutation
            relation :math:`[\x,\p]=i\hbar`.

    Returns:
        complex: the density matrix element
    """
    rpt = i + j
    beta = Beta(mu, hbar=hbar)
    A = Amat(cov, hbar=hbar)
    if np.linalg.norm(beta) < tol:
        # no displacement
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            A_rpt = reduction(A, rpt)
            haf = hafnian(A_rpt)
        else:
            haf = hafnian_repeated(A, rpt)
    else:
        # replace the diagonal of A with gamma
        gamma = beta.conj() - A @ beta
        if np.prod([k + 1 for k in rpt]) ** (1 / len(rpt)) < 3:
            A_rpt = reduction(A, rpt)
            np.fill_diagonal(A_rpt, reduction(gamma, rpt))
            haf = hafnian(A_rpt, loop=True)
        else:
            haf = hafnian_repeated(A, rpt, mu=gamma, loop=True)

    if include_prefactor:
        haf *= prefactor(mu, cov, hbar=hbar)

    return haf / np.sqrt(np.prod(fac(rpt)))