1. Regular canonical systems;Buchi;Arch. Math. Logik Grundlagenforsch.,1964
2. Grammars without Syntactic Variables;Buttlemann;Technical Report 74-1,1974
3. The sequence equivalence problem for OS systems is decidable;Ehrenfeucht;manuscript,1978
4. Pure Grammars and Pure Languages;Gabrielian;Dept. of Computer Research Report CSRR 2027,1970
5. A simple variable free CF grammar generating a non regular language;Hagauer;Bull. EATCS,1978