def filter(df, condition):
    """Returns entries in `df` where `condition` (a boolean DataFrame) is True.

    Args:
        df (:class:`pandas.DataFrame`): Our DataFrame.
        condition (:class:`pandas.DataFrame`): Boolean DataFrame used to filter `df`.

    Returns:
        :class:`pandas.DataFrame`: Entries in `df` where `condition` is True.
    """
    return df.loc[condition[condition].index]