def example_bsa() -> cw.BitstringAccumulator:
    """Test fixture to create an (empty) example BitstringAccumulator"""
    q0, q1 = cirq.LineQubit.range(2)
    setting = cw.InitObsSetting(
        init_state=cirq.KET_ZERO(q0) * cirq.KET_ZERO(q1), observable=cirq.X(q0) * cirq.Y(q1)
    )
    meas_spec = _MeasurementSpec(
        max_setting=setting, circuit_params={'beta': 0.123, 'gamma': 0.456}
    )
    bsa = cw.BitstringAccumulator(
        meas_spec=meas_spec,
        simul_settings=[
            setting,
            cw.InitObsSetting(init_state=setting.init_state, observable=cirq.X(q0)),
            cw.InitObsSetting(init_state=setting.init_state, observable=cirq.Y(q1)),
        ],
        qubit_to_index={q0: 0, q1: 1},
    )
    return bsa
Test fixture to create an (empty) example BitstringAccumulator
cirq-core/cirq/work/observable_measurement_data_test.py