code: public Key floor(Key key) {
  	if (isEmpty())
  	    throw new NoSuchElementException("called floor() with empty symbol table");
  	RedBlackTreeNode<Key, Value> x = floor(root, key);
  	if (x == null)
  	    return null;
  	else
  	    return x.getKey();
      }
docs: Returns the largest key in the symbol table less than or equal to
  <code>key</code>.
  
  @param key
  the key
  @return the largest key in the symbol table less than or equal to
  <code>key</code>
  @throws NoSuchElementException
  if there is no such key