Merges:
 6 +  7 -> 10 (modularity 0.00)
 2 +  4 -> 11 (modularity -0.07)
 8 + 10 -> 12 (modularity -0.04)
 3 + 11 -> 13 (modularity 0.02)
 9 + 12 -> 14 (modularity 0.08)
 1 + 13 -> 15 (modularity 0.16)
 0 + 15 -> 16 (modularity 0.25)
 5 + 14 -> 17 (modularity 0.35)
16 + 17 -> 18 (modularity 0.45)