minIndex = i;
          }
        }
        Optional<BinaryNode> leftChild = createTreap(keys.subList(0, minIndex));
        Optional<BinaryNode> rightChild = createTreap(keys.subList(minIndex + 1, keys.size()));
        return Optional.of(new BinaryNode(keys.get(minIndex), leftChild, rightChild));
      }
    };

    abstract Optional<BinaryNode> createTree(int size, Random rng);
  }

            minIndex = i;
          }
        }
        Optional<BinaryNode> leftChild = createTreap(keys.subList(0, minIndex));
        Optional<BinaryNode> rightChild = createTreap(keys.subList(minIndex + 1, keys.size()));
        return Optional.of(new BinaryNode(keys.get(minIndex), leftChild, rightChild));
      }
    };

    abstract Optional<BinaryNode> createTree(int size, Random rng);
  }

 * return an {@code Iterable<? extends N>}, then you can use a lambda to perform a more general
 * transformation:
 *
 * <pre>{@code
 * someGraphAlgorithm(startNode, node -> ImmutableList.of(node.leftChild(), node.rightChild()));
 * }</pre>
 *
 * <p>Graph algorithms that need additional capabilities (accessing both predecessors and
 * successors, iterating over the edges, etc.) should declare their input to be of a type that

   *
   * <p><b>Note on binary trees</b>
   *
   * <p>This method can be used to traverse over a binary tree. Given methods {@code
   * leftChild(node)} and {@code rightChild(node)}, this method can be called as
   *
   * <pre>{@code
   * Traverser.forTree(node -> ImmutableList.of(leftChild(node), rightChild(node)));
   * }</pre>
   *
   * @param tree {@link SuccessorsFunction} representing a directed acyclic graph that has at most