4 vertices, 10 edges
source=0 sink=3

=== DIMACS output ===
c DIMACS max-flow file generated from boost::write_dimacs_max_flow
p max 4 10
n 1 s
n 4 t
a 1 2 10
a 1 3 20
a 2 1 0
a 2 3 5
a 2 4 15
a 3 1 0
a 3 2 0
a 3 4 10
a 4 2 0
a 4 3 0
