1. Welsh, A.: ‘A database programming language: definition, implementation and correctness proofs’, 1984, Ph.D., Department of Computer Science, Manchester University, Manchester, England
2. Senko, M.E.: ‘The DDL in the context of a multilevel structured description: DIAM II with Foral’, Douque, B.C.M., Nijssen, G.M., (North-HollandData base description, 1975), p. 239–257
3. McLeod, I.: ‘SEQUEL as a language for document retrieval’, (Elsevier North-HollandThe AUERBACH annual 1980 best computing papers, 1980), p. 375–390
4. ‘An ALGOL-based associative language’, August, 1969 p. 439–449