code: final TreeMapEntry<K,V> getFirstEntry() {
          TreeMapEntry<K,V> p = root;
          if (p != null)
              while (p.left != null)
                  p = p.left;
          return p;
      }
docs: Returns the first Entry in the TreeMap (according to the TreeMap's
  key-sort function).  Returns null if the TreeMap is empty.