code: func (l *State) Context() (int, bool, error) {
  	if l.callInfo.isCallStatus(callStatusYielded) {
  		return l.callInfo.context, l.callInfo.shouldYield, l.callInfo.error
  	}
  	return 0, false, nil
  }
docs: // Context is called by a continuation function to retrieve the status of the
  // thread and context information. When called in the origin function, it
  // will always return (0, false, nil). When called inside a continuation function,
  // it will return (ctx, shouldYield, err), where ctx is the value that was
  // passed to the callee together with the continuation function.
  //
  // http://www.lua.org/manual/5.2/manual.html#lua_getctx