docs: function lub(tree, node, key, comp, high) {
      if (node === tree.z) {
          return high;
      }
      else if (comp(key, node.key) <= 0) {
          // The node key is a valid upper bound, but may not be the least.
          return minNode(node, lub(tree, node.l, key, comp, high), comp);
      }
      else {
          // Take the right link in search of bigger keys.
          return lub(tree, node.r, key, comp, high);
      }
  }