code: func (i *Index) Fields() []*Field {
  	i.mu.RLock()
  	defer i.mu.RUnlock()
  
  	a := make([]*Field, 0, len(i.fields))
  	for _, f := range i.fields {
  		a = append(a, f)
  	}
  	sort.Sort(fieldSlice(a))
  
  	return a
  }
docs: // Fields returns a list of all fields in the index.