code: def unique(seq):
      
      seen = set()
      return (x for x in seq if x not in seen and not seen.add(x))
docs: Returns a generator yielding unique sequence members in order
  
      A set by itself will return unique values without any regard for order.
  
      >>> list(unique([1, 2, 3, 2, 2, 4, 1]))
      [1, 2, 3, 4]