H. Fernau, M Holzer, R. Freund:
"Hybrid Modes in Cooperating Distributed Grammar Systems: Combining the t-mode with the Modes <=k and =k";
Theoretical Computer Science, 299 (2003), 633 - 662.
http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC04404611