... and radius 2, with endpoints b, c and central angleπ3. Similarly arcs A band A care defined. In other words, the points a, b, c and the arcs A a, A b, A cform the vertices and edges of a ... We define an upper boundary of D as an arc,which is a common boundary of D and D (in case that a 1= l1we define the upperboundary as {l1}). Similarly we define a lower boundary of D. For ... Pavel Valtr and Jan Kratochv´ıl who led the seminar under whichthis article has originated. We would also like to thank to Peter Bella, Josef Cibulka andMarek Tesaˇr, who were participants of the...