http://doi.acm.org/10.1145/800227.806871 db/conf/sigmod/McLeodS80.html Dennis McLeod John Miles Smith 19-25 PINGREE1980/P019.pdf 1980 conf/sigmod/80p Workshop on Data Abstraction, Databases and Conceptual Modelling db/conf/sigmod/pingree80.html#McLeodS80