ImmutableSet<N> validated = validate(startNodes);
    return () -> newTraversal().postOrder(validated.iterator());
  }

  abstract Traversal<N> newTraversal();

  @SuppressWarnings("CheckReturnValue")
  private ImmutableSet<N> validate(Iterable<? extends N> startNodes) {
    ImmutableSet<N> copy = ImmutableSet.copyOf(startNodes);
    for (N node : copy) {

import org.jspecify.annotations.Nullable;

/**
 * Unit test for {@link NullPointerTester}.
 *
 * @author Kevin Bourrillion
 * @author Mick Killianey
 */
@SuppressWarnings({
  "CheckReturnValue",
  "unused", // many methods tested reflectively -- maybe prefer local @Keep annotations?
})
@NullUnmarked
public class NullPointerTesterTest extends TestCase {

  /** Non-NPE RuntimeException. */