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.

c LEMON max flow problem

p max 7 9

n 1 s

n 7 t

a 1 2 3

a 1 3 2

a 1 4 1

a 2 5 3

a 3 5 2

a 3 7 5

a 3 6 3

a 4 6 1

a 5 7 2

a 6 7 4