/* An interface for updating a key-value index.
 */
@value interface KVWriter<#k,#v|> {
  set (#k,#v) -> (KVWriter<#k,#v>)
  remove (#k) -> (KVWriter<#k,#v>)
}

/* An interface for looking up values from a key-value index.
 */
@value interface KVReader<#k|#v> {
  get (#k) -> (optional #v)
}

/* An AVL implementation of KVWriter and KVReader.
 */
concrete Tree<#k,#v> {
  refines KVWriter<#k,#v>
  refines KVReader<#k,#v>
  #k defines LessThan<#k>

  // Creates a new Tree, e.g., Tree$new().
  @type new () -> (Tree<#k,#v>)

  // Normally you do not need to redeclare functions inherited from interfaces.
  // The two redeclarations below are used to refine the return type from
  // KVWriter to Tree.

  @value set (#k,#v) -> (Tree<#k,#v>)
  @value remove (#k) -> (Tree<#k,#v>)
}