1. Department of Mathematics, Southern Illinois University, Carbondale, IL, USA
2. Department of Mathematics, Kazan Federal University, Kazan, Republic of Tatarstan, Russian Federation
3. Department of Mathematics, George Washington University, Washington, D.C., USA
4. Department of Mathematics, University of Notre Dame, Notre Dame, IN, USA
5. Department of Mathematics, University of Portland, Portland, OR, USA
6. Department of Mathematical Logic, Sofia University, Sofia, Bulgaria
7. Department of Mathematical Logic, Faculty of Mathematics and Informatics, Sofia University, Sofia, Bulgaria