code: func Stack(l *State, level int) (f Frame, ok bool) {
  	if level < 0 {
  		return // invalid (negative) level
  	}
  	callInfo := l.callInfo
  	for ; level > 0 && callInfo != &l.baseCallInfo; level, callInfo = level-1, callInfo.previous {
  	}
  	if level == 0 && callInfo != &l.baseCallInfo { // level found?
  		f, ok = callInfo, true
  	}
  	return
  }
docs: // Stack gets information about the interpreter runtime stack.
  //
  // It returns a Frame identifying the activation record of the
  // function executing at a given level. Level 0 is the current running
  // function, whereas level n+1 is the function that has called level n (except
  // for tail calls, which do not count on the stack). When there are no errors,
  // Stack returns true; when called with a level greater than the stack depth,
  // it returns false.