Adds a single function after a function that already exists in the
        pipeline.

def after(self, existing_fn, new_fn):
        """Adds a single function after a function that already exists in the
        pipeline."""
        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)