def _stop_times_definitions(df):
    """
    Append GTFS definitions for stop time columns to stop time dataframe

    Parameters
    ----------
    df : pandas:DataFrame
        stop time dataframe

    Returns
    -------
    df : pandas.DataFrame
    """
    if 'pickup_type' in df.columns:
        stop_times_pickup_type = {(0): 'Regularly Scheduled', (1):
            'Not available', (2): 'Phone arrangement only', (3):
            'Driver arrangement only'}
        df['pickup_type_desc'] = df['pickup_type'].map(stop_times_pickup_type
            .get)
    if 'drop_off_type' in df.columns:
        stop_times_drop_off_type = {(0): 'Regularly Scheduled', (1):
            'Not available', (2): 'Phone arrangement only', (3):
            'Driver arrangement only'}
        df['drop_off_type_desc'] = df['drop_off_type'].map(
            stop_times_drop_off_type.get)
    if 'timepoint' in df.columns:
        stop_times_timepoint = {'': 'Exact times', (0): 'Approximate times',
            (1): 'Exact times'}
        df['timepoint_desc'] = df['timepoint'].map(stop_times_timepoint.get)
    return df
