def f_wildcard(self, wildcard='$', run_idx=None):
        """#TODO"""
        if run_idx is None:
            run_idx = self.v_idx
        wildcards = self._wildcard_keys[wildcard]
        try:
            return self._wildcard_cache[(wildcards, run_idx)]
        except KeyError:
            translation = self._wildcard_functions[wildcards](run_idx)
            self._wildcard_cache[(wildcards, run_idx)] = translation
            return translation