Iterates over all of the referreds, and then iterates over all of the
    referrers that refer to each one.

    Fairly item instantiation heavy.

def benchmark(store, n=10000):
    """
    Iterates over all of the referreds, and then iterates over all of the
    referrers that refer to each one.

    Fairly item instantiation heavy.
    """
    R = Referrer

    for referred in store.query(Referred):
        for _reference in store.query(R, R.reference == referred):
            pass