Scala: case object vs case class

http://stackoverflow.com/questions/5270752/difference-between-case-object-and-object

http://stackoverflow.com/questions/2312881/what-is-the-difference-between-scalas-case-class-and-class

http://stackoverflow.com/questions/4067792/why-are-case-objects-serializable-and-case-classes-not

http://samedi9999.wordpress.com/2011/03/15/scala-case-objects-as-enums/