'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'

def custom(self, ref, context=None):

    if (ref is None):
        return True
    else:
        return (not self.builtin(ref, context))
