http://doi.acm.org/10.1145/382272.382415 db/journals/sigmod/HeytensN89.html Michael L. Heytens Rishiyur S. Nikhil GESTALT: An Expressive Database Programming System. 54-67 sigmodR/18-1/P054.pdf 1989 18 SIGMOD Record 1 db/journals/sigmod/sigmod18.html#HeytensN89