def run(self, tokens):
        """Runs the current list of functions that make up the pipeline against
        the passed tokens."""
        for fn in self._stack:
            results = []
            for i, token in enumerate(tokens):
                # JS ignores additional arguments to the functions but we
                # force pipeline functions to declare (token, i, tokens)
                # or *args
                result = fn(token, i, tokens)
                if not result:
                    continue
                if isinstance(result, (list, tuple)):  # simulate Array.concat
                    results.extend(result)
                else:
                    results.append(result)
            tokens = results

        return tokens