1. Department of Teaching, Learning and Culture, 4232 Texas A&M University, 308 Harrington Tower, College Station, Texas 77843-4232, USA
2. Faculty of Mathematics and Physics, Charles University, Sokolovská 83, 186 75 Praha, Czech Republic
3. Department of Mathematics, National University of Singapore, Block S17, 10 Lower Kent Ridge Road, Singapore 119076, Singapore