flow:grid may have "holes" - cells marked by 0 (zero)
flow:grid(1 1 0 0 0 0,
          0 2 2 2 0 0,
          0 4 3 3 3 3,
          0 4 5 0 0 8,
          0 4 5 6 0 9,
         10 4 5 6 7 0);
  
1
2
3
4
5
6
7
8
9
10