code: public Key select(int k) {
  	if (k < 0 || k >= size(root))
  	    throw new IllegalArgumentException();
  	RedBlackTreeNode<Key, Value> x = select(root, k);
  	return x.getKey();
      }
docs: Return the kth smallest key in the symbol table.
  
  @param k
  the order statistic
  @return the kth smallest key in the symbol table
  @throws IllegalArgumentException
  unless <code>k</code> is between 0 and
  <em>N</em> &minus; 1