def _trips_definitions(df):
    """
    Append GTFS definitions for trip columns to trip dataframe

    Parameters
    ----------
    df : pandas:DataFrame
        trip dataframe

    Returns
    -------
    df : pandas.DataFrame
    """
    if 'bikes_allowed' in df.columns:
        trips_bikes_allowed = {(1): 'can accommodate at least one bicycle',
            (2): 'no bicycles are allowed on this trip'}
        df['bikes_allowed_desc'] = df['bikes_allowed'].map(trips_bikes_allowed
            .get)
    if 'wheelchair_accessible' in df.columns:
        trips_wheelchair_accessible = {(1):
            'can accommodate at least one rider in a wheelchair', (2):
            'no riders in wheelchairs can be accommodated on this trip'}
        df['wheelchair_accessible_desc'] = df['wheelchair_accessible'].map(
            trips_wheelchair_accessible.get)
    return df
