code: public Key floor(Key k) {
  		if (st.containsKey(k)) return k;
  
  		// does not include key if present (!)
  		SortedMap<Key, Value> head = st.headMap(k);
  		if (head.isEmpty()) return null;
  		else return head.lastKey();
  	}
docs: Return the largest key in the table <= k.