[1] Univ Witwatersrand, Sch Math, 1 Jan Smuts Ave, ZA-2000 Johannesburg, South Africa
[2] King Abdulaziz Univ, Res Grp Algebra Struct & Applicat, Jeddah, Saudi Arabia
[3] Max Planck Inst Math, Vivatsgasse 7, D-53111 Bonn, Germany
[4] Univ Ostrava, Dept Math, 30 Dubna 22, CZ-70103 Ostrava 1, Czech Republic