Improvement:

The item sets are written in the order sorted by the labels.

It solves the problem if we read a graph from a file and
then write it back then the nodes will be reversed.
It can be switched off with the LemonWriter interface.

 @nodeset

 label

 0

 1

 2

 3

 4

 5

 6

 @edgeset

         weight

 0   3   1

 0   4   1

 1   5   1

 5   2   1

 6   2   1

 0   1   1

 2   1   1

 3   4   1

 4   5   1

 5   6   1

 @end