lua_upvaluejoin

lua_upvaluejoin[-0, +0, –]

void lua_upvaluejoin (lua_State *L, int funcindex1, int n1,
                                    int funcindex2, int n2);

Make the n1-th upvalue of the Lua closure at index funcindex1 refer to the n2-th upvalue of the Lua closure at index funcindex2.