code: func (f Fields) SortedEntries() (s []*FieldEntry) {
  	if len(f) == 0 {
  		return nil
  	}
  	s = make([]*FieldEntry, 0, len(f))
  	for k, v := range f {
  		s = append(s, &FieldEntry{k, v})
  	}
  	sort.Sort(fieldEntrySlice(s))
  	return
  }
docs: // SortedEntries processes a Fields object, pruning invisible fields, placing
  // the ErrorKey field first, and then sorting the remaining fields by key.