1. Departamento de Matemáticas, Universidad del Valle, Calli, Colombia
2. School of Mathematics, University of the Witwatersrand, Johannesburg, South Africa
3. Research Group in Algebraic Structures and Applications, King Abdulaziz University, Jeddah, Saudi Arabia
4. Max Planck Institute for Software Systems, Saarbrücken, Germany
5. Centro de Ciencias Matemáticas, UNAM, Morelia, Mexico