1. Universality of data retrieval languages;Aho,1979
2. Apt, K., and Plotkin G. A Cook's Tour of Countable Nondeterminism, ICALP '81.
3. Admissible Sets & Structures;Barwise,1975
4. Computable queries for relational data bases;Chandra;JCSS,1980
5. Structure and Complexity of Relational Queries;Chandra;JCSS,1982