code: func (tag *Tag) CommonID(description string) string {
  	var ids map[string]string
  	if tag.version == 3 {
  		ids = V23CommonIDs
  	} else {
  		ids = V24CommonIDs
  	}
  	if id, ok := ids[description]; ok {
  		return id
  	}
  	return description
  }
docs: // CommonID returns ID3v2.3 or ID3v2.4 (in appropriate to version of Tag) frame ID
  // from given description.
  // For example, CommonID("Language") will return "TLAN".
  // If it can't find the ID with given description, it returns the description.
  //
  // All descriptions you can find in file common_ids.go
  // or in id3 documentation (for fourth version: http://id3.org/id3v2.4.0-frames;
  // for third version: http://id3.org/id3v2.3.0#Declared_ID3v2_frames).