[1] S. Peters R. W. Ritchie: On the generative power of transformational grammars (mimeographed).
[2] N. Chomsky: Aspects of the Theory of Syntax. MIT 1965.
[3] S. C. Kleene: Introduction to Metamatematics. N. Y. 1965.
[4] K. Čulík:
On some transformations in context-free grammars and languages. Czech. Math. Journ. 17 (92), (1967), 278-311.
MR 0214410
[5] K. Čulík: On the ordered rooted trees used in theory of laaguages. Théorie des graphes-journées internationales ďétude, Rome, juillet 1966. Dunod, Paris-Gordon and Breach, New York 1967, 69-76.