Lyle, Don M. "A hierarchy of high order languages for systems programming" pp73-78

[LSI 1971] (1971) [ACM] SIGPLAN Notices 6(10) October 1971 Proceedings of the SIGPLAN symposium on Languages for system implementation 1971, Lafayette, Indiana, United States; October, 1971  

Languages mentioned DCALGOL {1970} Data Communications ALGOL ESPOL {1968} Executive System Problem Oriented Language Extended ALGOL {1964}