def is_bus(df):
    """Return if vehicle is a bus, per NHTSA convention."""
    yr = df['YEAR']
    body = df['BODY_TYP']
    return yr.between(1975, 1981) & body.between(25, 29) | yr.between(1982,
        1990) & body.between(30, 39) | (1991 <= yr) & body.between(50, 59)
