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

 * 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