Lijun Dong Jiafu Xu Formal semantics of some functional constructs of the software requirements definition language NDRDL. 642-645 1997 conf/compsac/1997 COMPSAC http://doi.ieeecomputersociety.org/10.1109/CMPSAC.1997.625086 db/conf/compsac/compsac1997.html#DongX97