http://doi.acm.org/10.1145/984409.984411 db/journals/sigmod/Mooers69a.html Calvin N. Mooers Data Decriptive Languages. 31-36 sigmodR/01-1/P031.pdf 1969 1 FDT - Bulletin of ACM SIGFIDET 1 db/journals/sigmod/sigmod1.html#Mooers69a