Crossroads in flatland

Note: Professor Pach's number: [138]; Also in: MTA Kozgy{\Hu}lesi El{\Ho}adasok, J. Poto ed., Hungarian Academy of Sciences, Budapest, 1999, pp. 131-138, in Hungarian Reference DCG-CHAPTER-2008-019 Record created on 2008-11-18, modified on 2017-05-12