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.

 p max 10 17

 n 2 s

 n 9 t

 a 1 2 20

 a 1 3 0

 a 2 2 3

 a 2 3 8

 a 2 4 8

 a 3 6 5

 a 4 3 5

 a 4 6 5

 a 4 7 5

 a 5 4 3

 a 6 8 3

 a 6 7 10

 a 6 9 10

 a 7 9 8

 a 9 10 20

 a 9 2 5

 a 10 6 5