code: public int addUniqueElement(int element, int treap) {
  		int treap_;
  		if (treap == -1) {
  			if (m_defaultTreap == nullNode())
  				m_defaultTreap = createTreap(-1);
  			treap_ = m_defaultTreap;
  		} else {
  			treap_ = treap;
  		}
  
  		return addElement_(element, 1, treap_);
  	}
docs: the already existing element equal to element.