def within_set(df, items=None):
    """
    Assert that df is a subset of items

    Parameters
    ==========
    df : DataFrame
    items : dict
      mapping of columns (k) to array-like of values (v) that
      ``df[k]`` is expected to be a subset of

    Returns
    =======
    df : DataFrame
    """
    for k, v in items.items():
        if not df[k].isin(v).all():
            bad = df.loc[~df[k].isin(v), k]
            raise AssertionError('Not in set', bad)
    return df
