-- Athens University of Economics and Business -- 	http://www.aueb.gr
	     Department of Informatics			http://www.cs.aueb.gr

Ntanasis Periklis - 3070130
Chatzipetros Mike - 3070175

see http://docs.google.com/Doc?docid=0Aa_ksH7errvWZGNtcmQ1M2dfMWZ4bmt6cmc4&hl=el 
to understand how find_successor and closest_preceding_node works