1. The Theory of Parsing, Translation, and Compiling, Vol. I: Parsing;Aho,1972
2. On the covering and reduction problems for context-free grammars;Gray;J. ACM,1972
3. Überdeckung Kontextfreier Grammatiken;Laufkötter,1979
4. Grammar functors and covers: From non-left-recursive to Greibach normal form grammars;Nijholt;BIT,1979
5. Context-free Grammars: Covers, Normal Forms, and Parsing;Nijholt,1980