docs: def remove(self, w):
          """
          Removes a word from the vocab. The indices are unchanged.
          """
          if w not in self.f2i:
              raise ValueError("'{}' does not exist.".format(w))
  
          if w in self.reserved:
              raise ValueError("'{}' is one of the reserved words, and thus"
                               "cannot be removed.".format(w))
  
          index = self.f2i[w]
          del self.f2i[w]
          del self.i2f[index]
  
          self.words.remove(w)