{[a,{[1,2],[2,3],[2,4],[7,8],[7,9],[8,10],[8,11],[9,12]}],
 [b,{[3,5],[10,13],[12,15]}],
 [c,{[4,6],[11,14]}]}
{[1,7],[1,8],[1,9],
 [2,7],[2,8],[2,9],
 [3,10],[3,12],
 [4,11],
 [5,13],[5,14],[5,15],
 [6,13],[6,14],[6,15]}