[(0,1),(0,2),(1,3),(3,4),(1,5),(4,6),(6,7),(5,7),(3,8),(5,9),(9,10),(2,11),(10,12),(4,13),(10,14),(9,15),(2,16),(6,17),(8,18),(7,19),(8,20)]
[(0,1),(1,2),(0,3),(3,4),(2,5),(5,6),(6,7),(4,7),(1,8),(4,9),(9,10),(8,11),(5,12),(9,13),(10,14),(2,15),(8,16),(6,17),(7,18),(3,19),(10,20)]
