function did not work correctly

token_list = ['a', 'b', 'c']

contradictions = {
    'a': set(['b']),
    'b': set(['a']),
    'c': set()
}

def solve(tokens, contradictions):
   if not tokens:
      yield set()
   else:
      tokens = set(tokens)
      t = tokens.pop()
      for solution in solve(tokens - contradictions[t], contradictions):
         yield solution | set([t])
      if contradictions[t] & tokens:
         for solution in solve(tokens, contradictions):
            if contradictions[t] & solution:
               yield solution

print list(solve(token_list, contradictions))