Your search returned the following 2 documents:

2. Two Versus One Index Register and Modifiable Versus Non-modifiable Programs
   Kurt Mehlhorn and Wolfgang J. Paul
   In: Automata, languages and programming (ICALP-89) : 16th international colloquium, Stresa, Italy, 1989, 603-609
   
1. $k$ versus $k+1$ Index Registers and Modifiable versus Non-modifiable Programs
   Kurt Mehlhorn, Wolfgang J. Paul, and C Uhrig
   Information and Computation 101 (1): 123-129, 1992
   [PDF: Download: KVersusPlusOne.pdf]