How to use the openfermion.count_qubits function in openfermion

def _generate_qubits(self) -> Sequence[cirq.Qid]:
        """Produce qubits that can be used by the ansatz circuit."""
        return cirq.LineQubit.range(openfermion.count_qubits(self.hamiltonian))

def _generate_qubits(self) -> Sequence[cirq.Qid]:
        """Produce qubits that can be used by the ansatz circuit."""
        return cirq.LineQubit.range(openfermion.count_qubits(self.hamiltonian))

def _generate_qubits(self) -> Sequence[cirq.Qid]:
        """Produce qubits that can be used by the ansatz circuit."""
        return cirq.LineQubit.range(openfermion.count_qubits(self.hamiltonian))

to calculate the bitmask for.
        n_qubits (int): The number of qubits (modes) in the system. If not
                        specified, defaults to the maximum of any term in
                        fermion_term_list.

    Returns:
        An n_qubits x len(fermion_term_list) boolean numpy array of whether
        each term acts on the given mode index.

    Raises:
        ValueError: if n_qubits is too small for the given terms.
    """
    if n_qubits is None:
        n_qubits = 0
        for term in fermion_term_list:
            n_qubits = max(n_qubits, count_qubits(term))

    mask = numpy.zeros((n_qubits, len(fermion_term_list)), dtype=bool)

    for term_number, term in enumerate(fermion_term_list):
        actions = term.terms
        for action in actions:
            for single_operator in action:
                mode = single_operator[0]
                try:
                    mask[mode][term_number] = True
                except IndexError:
                    raise ValueError('Bad n_qubits: must be greater than '
                                     'highest mode in any FermionOperator.')

    return mask

error_operator: The second-order Trotter error operator.

    Notes:
        Follows Eq 9 of Poulin et al., arXiv:1406.4920, applied to the
        Trotter step detailed in Kivlichan et al., arxiv:1711.04789.
    """
    single_terms = numpy.array(
        simulation_ordered_grouped_low_depth_terms_with_info(
            hamiltonian,
            external_potential_at_end=external_potential_at_end)[0])

    # Cache the halved terms for use in the second commutator.
    halved_single_terms = single_terms / 2.0

    term_mode_mask = bit_mask_of_modes_acted_on_by_fermionic_terms(
        single_terms, count_qubits(hamiltonian))

    error_operator = FermionOperator.zero()

    for beta, term_beta in enumerate(single_terms):
        modes_acted_on_by_term_beta = set()
        for beta_action in term_beta.terms:
            modes_acted_on_by_term_beta.update(
                set(operator[0] for operator in beta_action))

        beta_mode_mask = numpy.logical_or.reduce(
            [term_mode_mask[mode] for mode in modes_acted_on_by_term_beta])

        # alpha_prime indices that could have a nonzero commutator, i.e.
        # there's overlap between the modes the corresponding terms act on.
        valid_alpha_primes = numpy.where(beta_mode_mask)[0]

with the kinetic energy T first (order='T+V') or with
                     the potential energy V first (order='V+T').

    Returns:
        error_operator: The second-order Trotter error operator.

    Notes:
        The second-order split-operator Trotter error is calculated from the
        double commutator [T, [V, T]] + [V, [V, T]] / 2 when T is simulated
        before V (i.e. exp(-iTt/2) exp(-iVt) exp(-iTt/2)), and from the
        double commutator [V, [T, V]] + [T, [T, V]] / 2 when V is simulated
        before T, following Equation 9 of "The Trotter Step Size Required for
        Accurate Quantum Simulation of Quantum Chemistry" by Poulin et al.
        The Trotter error operator is then obtained by dividing by 12.
    """
    n_qubits = count_qubits(hamiltonian)

    potential_terms, kinetic_terms = (
        diagonal_coulomb_potential_and_kinetic_terms_as_arrays(hamiltonian))

    # Cache halved potential and kinetic terms for the second commutator.
    halved_potential_terms = potential_terms / 2.0
    halved_kinetic_terms = kinetic_terms / 2.0

    # Assign the outer term of the second commutator based on the ordering.
    outer_potential_terms = (halved_potential_terms if order == 'T+V' else
                             potential_terms)
    outer_kinetic_terms = (halved_kinetic_terms if order == 'V+T' else
                           kinetic_terms)

    potential_mask = bit_mask_of_modes_acted_on_by_fermionic_terms(
        potential_terms, n_qubits)