code: public Tree<K, V> lookup(Tree<K, V> tree, K x) {
          if (tree == null)
              return null;
  
          int cmp = ordering.compare(x, tree.getKey(kf));
  
          if (cmp < 0) return lookup(tree.getLeft(), x);
          else if (cmp > 0) return lookup(tree.getRight(), x);
          else return tree;
      }
docs: TODO: Remove recursion @tailrec