"Sometimes" and "not never" revisited

E. Allen Emerson, Joseph Y. Halpern
1983 Proceedings of the 10th ACM SIGACT-SIGPLAN symposium on Principles of programming languages - POPL '83  
doi:10.1145/567067.567081 dblp:conf/popl/EmersonH83 fatcat:peexmerl2zc7raffdcnn5tn5qe