code: func UpValueJoin(l *State, f1, n1, f2, n2 int) {
  	u1 := l.upValue(f1, n1)
  	u2 := l.upValue(f2, n2)
  	*u1 = *u2
  }
docs: // UpValueJoin makes the n1-th upvalue of the Lua closure at index f1 refer to
  // the n2-th upvalue of the Lua closure at index f2.