1. On the decidability of the sequence equivalence problem for DOLsystems;Culik;Theor. Computer Sci.,1977
2. also Res. Rep. CS-75-24, Dept. of Computer Science, University of Waterloo Waterloo
3. Ehrenfeucht, A., and Rozenberg, G. (1974) , private communication.
4. Free groups in Lindenmayer systems;Johansen,1974