Heath, I. J. (1971). "Unacceptable file operations in a relational data base". Proceedings of the 1971 ACM SIGFIDET (now SIGMOD) Workshop on Data Description, Access and Control - SIGFIDET '71. pp. 19–33. doi:10.1145/1734714.1734717. S2CID22069259. cited in:
Ronald Fagin and Moshe Y. Vardi (1986). "The Theory of Data Dependencies - A Survey". In Michael Anshel and William Gewirtz (ed.). Mathematics of Information Processing: [short Course Held in Louisville, Kentucky, January 23-24, 1984]. American Mathematical Soc. p. 23. ISBN978-0-8218-0086-7.
Heath, I. J. (1971). "Unacceptable file operations in a relational data base". Proceedings of the 1971 ACM SIGFIDET (now SIGMOD) Workshop on Data Description, Access and Control - SIGFIDET '71. pp. 19–33. doi:10.1145/1734714.1734717. S2CID22069259. cited in:
Ronald Fagin and Moshe Y. Vardi (1986). "The Theory of Data Dependencies - A Survey". In Michael Anshel and William Gewirtz (ed.). Mathematics of Information Processing: [short Course Held in Louisville, Kentucky, January 23-24, 1984]. American Mathematical Soc. p. 23. ISBN978-0-8218-0086-7.
Heath, I. J. (1971). "Unacceptable file operations in a relational data base". Proceedings of the 1971 ACM SIGFIDET (now SIGMOD) Workshop on Data Description, Access and Control - SIGFIDET '71. pp. 19–33. doi:10.1145/1734714.1734717. S2CID22069259. cited in:
Ronald Fagin and Moshe Y. Vardi (1986). "The Theory of Data Dependencies - A Survey". In Michael Anshel and William Gewirtz (ed.). Mathematics of Information Processing: [short Course Held in Louisville, Kentucky, January 23-24, 1984]. American Mathematical Soc. p. 23. ISBN978-0-8218-0086-7.