Deheneffe , Claude and Hennebert, Henri "NUL: a Navigational User's Language for a network structured data base"

[ACM SIGMOD/PODS] (1976) [ACM SIGMOD/PODS] Proceedings of the 1976 SIGMOD/PODS Conference on Data : Abstraction, definition and structure June 02-04, 1976, Washington, D.C.  

Languages mentioned NUL {1976} Network database language