def query_weight(terms):
    """Return the weight for a set of query terms.

        'terms' is a sequence of all terms included in the query,
        although not terms with a not.  If a term appears more than
        once in a query, it should appear more than once in terms.

        Nothing is defined about what "weight" means, beyond that the
        result is an upper bound on document scores returned for the
        query.
        """
