1. J. Albert und L. Wegner, Languages with homomorphic replacements, University Karlsruhe, Report No. 88, (1980).
2. P.R.J. Asveld and J. Engelfriet, Iterated deterministic substitution, Acta Informatica 8, 3 (1977) 285–302.
3. P. Dembiński and J. Małuszyński, Two level grammars: CF-grammars with equation schemes, Proc. 6th ICALP, Graz, LNCS 71, (1979) 171–187.
4. A. Ehrenfeucht, G. Rozenberg and S. Skyum, A relationship between ETOL and EDTOL languages, Theoretical Computer Science 1 (1976) 325–330.
5. J. Engelfriet and E.M. Schmidt, IO and OI, Part I: Journal of Comp. System Sciences 15 (1977) 328–353); Part II: Journal of Comp. Syst. Sciences 16 (1978) 67–99.