code: func (w *newValueWriter) WriteField(address []string, value interface{}, computed bool) error {
  	// Fail the write if we have a non-nil value and computed is true.
  	// NewComputed values should not have a value when written.
  	if value != nil && computed {
  		return errors.New("Non-nil value with computed set")
  	}
  
  	if err := w.MapFieldWriter.WriteField(address, value); err != nil {
  		return err
  	}
  
  	w.once.Do(w.init)
  
  	w.lock.Lock()
  	defer w.lock.Unlock()
  	if computed {
  		w.computedKeys[strings.Join(address, ".")] = true
  	}
  	return nil
  }
docs: // WriteField overrides MapValueWriter's WriteField, adding the ability to flag
  // the address as computed.