private File testDir;
  private File tempDir;

  private final Set<File> filesToDelete = new HashSet<>();

  @Override
  protected void tearDown() {
    for (File file : filesToDelete) {
      if (file.exists()) {
        delete(file);
      }
    }
    filesToDelete.clear();
  }

  private File getTestDir() throws IOException {
    if (testDir != null) {
      return testDir;
    }

@ElementTypesAreNonnullByDefault
public abstract class TreeTraverser<T> {

  /**
   * Returns a tree traverser that uses the given function to navigate from a node to its children.
   * This is useful if the function instance already exists, or so that you can supply a lambda
   * expressions. If those circumstances don't apply, you probably don't need to use this; subclass
   * {@code TreeTraverser} and implement its {@link #children} method directly.
   *

@ElementTypesAreNonnullByDefault
public abstract class TreeTraverser<T> {

  /**
   * Returns a tree traverser that uses the given function to navigate from a node to its children.
   * This is useful if the function instance already exists, or so that you can supply a lambda
   * expressions. If those circumstances don't apply, you probably don't need to use this; subclass
   * {@code TreeTraverser} and implement its {@link #children} method directly.
   *