code: def after(self, existing_fn, new_fn):
          
          self.warn_if_function_not_registered(new_fn)
          try:
              index = self._stack.index(existing_fn)
              self._stack.insert(index + 1, new_fn)
          except ValueError as e:
              six.raise_from(BaseLunrException("Cannot find existing_fn"), e)
docs: Adds a single function after a function that already exists in the
          pipeline.