http://doi.acm.org/10.1145/984540.984544 db/journals/sigmod/Furtado84.html Antonio L. Furtado An Informal Approach to Formal Specifications. 45-54 sigmodR/14-1/P045.pdf 1984 14 SIGMOD Record 1 db/journals/sigmod/sigmod14.html#Furtado84