code: func (ns *Namespace) Merge(p2, p1 interface{}) (interface{}, error) {
  	merger, ok := p1.(pagesLanguageMerger)
  	if !ok {
  		return nil, fmt.Errorf("language merge not supported for %T", p1)
  	}
  	return merger.MergeByLanguageInterface(p2)
  }
docs: // Merge creates a union of pages from two languages.