def sample(n):
T = DiGraph()
alive = dict()
heights = list()
total = 0.0
for i in range(n):
alive[i] = 0.0
k = n
while k > 1:
event = exponential(1.0/binom(k, 2))
total += event
heights.append(total)
for c in alive.keys():
alive[c] += event
[a, b] = subset(alive.keys(), 2)
c = new_node(k)
alive[a]
alive[b]
T.add_edge(a, c, length = alive[a])
T.add_edge(b, c, length = alive[b])
del alive[a]
del alive[b]
alive[c] = 0.0
k -= 1
T.below = collapse(T)
T.heights = heights
return T
