1. Laboratoire Paul Painlevé; Université de Lille 1 et IUF; France
2. Laboratory of Algebraic Geometry; National Research University Higher School of Economics; 6 Usacheva Street Moscow 119048 Russia
3. Steklov Mathematical Institute; 8 Gubkina Street Moscow 117966 GSP-1 Russia
4. Department of Pure Mathematics; The University of Liverpool; Liverpool L69 3BX United Kingdom