[1] CTR MATH & COMP SCI,1009 AB AMSTERDAM,NETHERLANDS
[2] TECH UNIV EINDHOVEN,DEPT MATH & COMP SCI,5600 MB EINDHOVEN,NETHERLANDS
[3] CHRISTIAN ALBRECHTS UNIV KIEL,INST COMP SCI & PRACT MATH 2,W-2300 KIEL 1,GERMANY
[4] LEIDEN UNIV,DEPT COMP SCI,2300 RA LEIDEN,NETHERLANDS