1DEPARTMENT OF APPLIED MATHEMATICS, CHARLES UNIVERSITY, MALOSTRANSKE NAM. 25, 118 00 PRAHA 1, CZECH REPUBLIC
2DEPARTMENT OF APPLIED MATHEMATICS, CHARLES UNIVERSITY, MALOSTRANSKE NAM. 25, 118 00 PraHaA 1, CZECH REPUBLIC AND ZENTRUM MATHEMATIK (GRUPPE M9), TECHNISCHE UNIVERSITAT MUNCHEN, BOLTZMANNSTRASSE 3, D-85747 GARCHING BEI MUONCHEN, GERMANY
3DEPARTMENT OF COMBINATORICS AND OPTIMIZATION, UNIVERSITY OF WATERLOO, WATERLOO, ONTARIO, CANADA N2L 3G1