def iter(self, string, start=None, end=None):
  start = start if start is not None else 0
  end = end if end is not None else len(string)

  state = self.root
  for index, c in enumerate(string[start:end]):
   while c not in state.children:
    state = state.fail

   state = state.children.get(c, self.root)

   tmp = state
   output = []
   while tmp is not nil and tmp.output is not nil:
    output.append(tmp.output)
    tmp = tmp.fail

   if output:
    yield (index + start, output)