interface (

  Cursor-Type [Meta-Type (
    instance « (
      attributes « (
        Item-type Type,
        host Object
      ),
      instance « (
        receives « (
          ready?   (replies Boolean),
          at-item? (replies Boolean),
          get-item (
            requires « ([instance ← at-item?] else-throws no-item
            replies Item-type
          )
        )
      )
    )
  )],
  
  [Cursor (Item-type Type)] Cursor-Type,

  [Forward-Cursor (Item-type Type)] [Cursor-Type (
    supertypes « (Cursor (Item-type « Item-type)),
    instance « (
      receives « (
        at-end? (replies Boolean),
        move-to-next (
          requires « ([¬[instance ← at-end?]] else-throws at-end
        )
      )
    )
  )],

  [Two-Way-Cursor (Item-type Type)] [Cursor-Type (
    supertypes « (Forward-Cursor (Item-type « Item-type)),
    instance « (
      receives « (
        at-beginning? (replies Boolean),
        move-to-previous (
          requires « ([¬[instance ← at-beginning?]] else-throws at-beginning
        )
      )
    )
  )],

)

/* These are all abstract types. Cursor classes are implemented with the
   classes they are attached to. */