Miscellany

reachable = []
for i in range(n):
  reachable.append([False] * n)

for u in range(n):
  reachable[u][u] = True
  for v in topological_sort(u): 
    reachable[u][v] = True

for u in range(n):
  for v in range(n):
    if not reachable[u][v] and not reachable[v][u]:
      return False

return True