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