code: func (r Rules) Rule(tag language.Tag) *Rule {
  	t := tag
  	for {
  		if rule := r[t]; rule != nil {
  			return rule
  		}
  		t = t.Parent()
  		if t.IsRoot() {
  			break
  		}
  	}
  	base, _ := tag.Base()
  	baseTag, _ := language.Parse(base.String())
  	return r[baseTag]
  }
docs: // Rule returns the closest matching plural rule for the language tag
  // or nil if no rule could be found.