code: def custom(self, ref, context=None):
          
          if ref is None:
              return True
          else:
              return not self.builtin(ref, context)
docs: Get whether the specified reference is B{not} an (xs) builtin.
          @param ref: A str or qref.
          @type ref: (str|qref)
          @return: True if B{not} a builtin, else False.
          @rtype: bool