code: def extend_unique(seq, more):
      
      seen = set(seq)
      new = []
      for item in more:
          if item not in seen:
              seen.add(item)
              new.append(item)
  
      return seq + type(seq)(new)
docs: Return a new sequence containing the items in `seq` plus any items in
      `more` that aren't already in `seq`, preserving the order of both.