def is_van(df):
    """Return if vehicle is a van, per NHTSA convention."""
    yr = df['YEAR']
    body = df['BODY_TYP']
    return yr.between(1975, 1981) & (body == 51) | yr.between(1982, 1990
        ) & body.isin([40, 41, 48, 49]) | (1991 <= yr) & body.isin([20, 21,
        22, 24, 25, 28, 29])
