Polynomial-Time Implication Problems for Unary Inclusion Dependencies Stavros S. Cosmadakis Paris C. Kanellakis Moshe Y. Vardi 15-46 J. ACM January 1990 37 1 db/journals/jacm/jacm37.html#CosmadakisKV90 http://doi.acm.org/10.1145/78935.78937