Sorry I ripped the code from a mess and I forget the tree is "leaflized" as

tree = {
  'A': set(['B', 'C', 'D', 'E']),
  'B': set(['D', 'E']), 
  'C': set(),
  'D': set(),
  'E': set(),
}

I don't want to talk about the actual problem. I think sometime 
it's hard to give an "absolute" weight value as key, for this example,
is the relationship in graph. Then user have to "Copy and paste the recipe" to get a cmp function which should be already there. We are
all adults here, why don't SIMPLELY tell the newbie don't use cmp() use key() unless you know what you are doing.


Thanks for reply.