code: def benchmark(store, n=10000):
      
      R = Referrer
  
      for referred in store.query(Referred):
          for _reference in store.query(R, R.reference == referred):
              pass
docs: Iterates over all of the referreds, and then iterates over all of the
      referrers that refer to each one.
  
      Fairly item instantiation heavy.