http://doi.acm.org/10.1145/141356.141383 db/journals/sigmod/FankhauserKN91.html Peter Fankhauser Martin Kracker Erich J. Neuhold Semantic vs. Structural Resemblance of Classes. 59-63 sigmodR/20-4/P059.pdf 1991 20 SIGMOD Record 4 db/journals/sigmod/sigmod20.html#FankhauserKN91