code: def add(self, *args):
          
          for fn in args:
              self.warn_if_function_not_registered(fn)
              self._stack.append(fn)
docs: Adds new functions to the end of the pipeline.
  
          Functions must accept three arguments:
          - Token: A lunr.Token object which will be updated
          - i: The index of the token in the set
          - tokens: A list of tokens representing the set