if isinstance(func_def, types.FunctionType):
      if not hasattr(func_def, '_tfr_op_name'):
        # skip a non-composition function
        if optional:
          return (None, None)
        else:
          raise KeyError('OpDef does not exist: ' + f_name)
      op_name = getattr(func_def, '_tfr_op_name')
    elif not func_def:
      op_name = f_name
    else: