'Add a builtin and save the original.'

def add_builtin(self, key, value):

    bdict = builtin_mod.__dict__
    orig = bdict.get(key, BuiltinUndefined)
    if (value is HideBuiltin):
        if (orig is not BuiltinUndefined):
            self._orig_builtins[key] = orig
            del bdict[key]
    else:
        self._orig_builtins[key] = orig
        bdict[key] = value
