{ {1,3} , {2,4} }

	   --------
	   |      |
	   |A	  |B
	-----------------
	   |	  |	|
	   |	  |	|
	   |		|
	   |		|
	   --------------

	    A = {1,3}  

	    B = {2,4}


The shadow cannot close without 
altering the notation; this
notation is therefore undrawable.







{ {1,4}, {3,6}, {5,8}, {7,10}, {9,2} }

	   ---5----
	   |      |
	   |A	  |B
	------D----------
	   |	  |	|
	   |	  |	|
	   |  8---C	|
	   |		|
	   --------------

	    A = {1,4}

	    B = {3,6}

	    C = {7,10}

	    D = {9,2}

The points 5 and 8 cannot meet to
form the crossing {5,8} and thus
this notation is also undrawable.



Charilaos Aneziris, aneziris@hades.ifh.de Copyright 1995 COPYRIGHT STATEMENT Educational institutions are encouraged to reproduce and distribute these materials for educational use free of charge as long as credit and notification are provided. For any other purpose except educational, such as commercial etc, use of these materials is prohibited without prior written permission.