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);
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