code: func (w *Value) Close() error {
  	w.lock.Lock()
  	set := w.valueSet
  	w.lock.Unlock()
  
  	// If we haven't set the value, set it
  	if !set {
  		w.SetValue(ErrClosed)
  	}
  
  	// Done
  	return nil
  }
docs: // Close closes the value. This can never fail. For a definition of
  // "close" see the struct docs.