Data Structures, Algorithms, & Applications in Java
Chapter 15, Exercise 5

AbstractDataType DIndexedBSTree
{
   instances
      same as for IndexedBSTree except that the keys need not be distinct
   operations
      get(k): return any element with key k
      get(index): return the indexth element
      put(k,x): put element x with key k into the search tree
      remove(k): remove any element with key k and return it
      remove(index): remove the indexth element and return it
      ascend(): output all elements in nondecreasing order of key
}


We may use either the interface IndexedBSTree or define a new interface that extends IndexedBSTree, but contains no new methods. The new interface is given below. 
public interface DIndexedBSTree extends IndexedBSTree
   {}