input: Write a function to return unique words from a list of strings.
  assert unique_words(["This","is","a","list","of","strings","this","list","contains","duplicate","words"]) == ["This","is","a","list","of","strings","this","contains","duplicate","words"]
  assert unique_words(["apple","banana","apple","orange","banana","apple"]) == ["apple","banana","orange"]
  assert unique_words(["one","two","three","two","one","four"]) == ["one","two","three","four"]
output: def unique_words(words):
      seen = set()
      unique_list = []
      for word in words:
          if word not in seen:
              unique_list.append(word)
              seen.add(word)
      return unique_list