code: public KType indexGet(int index) {
      assert index >= 0 : "The index must point at an existing key.";
      assert index <= mask ||
             (index == mask + 1 && hasEmptyKey);
  
      return Intrinsics.<KType> cast(keys[index]);
    }
docs: Returns the exact value of the existing key. This method makes sense for sets
  of objects which define custom key-equality relationship.
  
  @see #indexOf
  
  @param index The index of an existing key.
  @return Returns the equivalent key currently stored in the set.
  @throws AssertionError If assertions are enabled and the index does
  not correspond to an existing key.