code: def add(T, w, i=0):
      
      if T is None:
          T = Trie_Node()
      if i == len(w):
          T.isWord = True
      else:
          T.s[w[i]] = add(T.s[w[i]], w, i + 1)
      return T
docs: :param T: trie
      :param string w: word to be added to T
      :returns: new trie consisting of w added into T
      :complexity: O(len(w))