/** Call this in a {@link TestSubtype} public method asserting subtype relationship. */
  final <T> T isSubtype(T sub) {
    Type returnType = method.getGenericReturnType();
    Type paramType = getOnlyParameterType();
    TestSubtype spec = method.getAnnotation(TestSubtype.class);
    assertWithMessage("%s is subtype of %s", paramType, returnType)
        .that(TypeToken.of(paramType).isSubtypeOf(returnType))

    Type subtype = new TypeCapture<TypedKeyMap<T>>() {}.capture();
    assertEquals(
        new TypeCapture<TypedKeyMap<String>>() {}.capture(),
        new TypeResolver()
            .where(
                new TypeCapture<Map<Integer, T>>() {}.capture(),
                new TypeCapture<Map<?, String>>() {}.capture())
            .resolveType(subtype));
  }

   * Returns the value the specified class is mapped to, or {@code null} if no entry for this class
   * is present. This will only return a value that was bound to this specific class, not a value
   * that may have been bound to a subtype.
   *
   * <p>{@code getInstance(Foo.class)} is equivalent to {@code
   * getInstance(TypeToken.of(Foo.class))}.
   */
  @CheckForNull
  <T extends @NonNull B> T getInstance(Class<T> type);

  /**

   * Returns the value the specified class is mapped to, or {@code null} if no entry for this class
   * is present. This will only return a value that was bound to this specific class, not a value
   * that may have been bound to a subtype.
   */
  @CheckForNull
  <T extends @NonNull B> T getInstance(Class<T> type);

  /**
   * Maps the specified class to the specified value. Does <i>not</i> associate this value with any
   * of the class's supertypes.

  /** Call this in a {@link TestSubtype} public method asserting subtype relationship. */
  final <T> T isSubtype(T sub) {
    Type returnType = method.getGenericReturnType();
    Type paramType = getOnlyParameterType();
    TestSubtype spec = method.getAnnotation(TestSubtype.class);
    assertWithMessage("%s is subtype of %s", paramType, returnType)
        .that(TypeToken.of(paramType).isSubtypeOf(returnType))

   * Returns the value the specified class is mapped to, or {@code null} if no entry for this class
   * is present. This will only return a value that was bound to this specific class, not a value
   * that may have been bound to a subtype.
   */
  @CheckForNull
  <T extends @NonNull B> T getInstance(Class<T> type);

  /**
   * Maps the specified class to the specified value. Does <i>not</i> associate this value with any
   * of the class's supertypes.

    Type subtype = new TypeCapture<TypedKeyMap<T>>() {}.capture();
    assertEquals(
        new TypeCapture<TypedKeyMap<String>>() {}.capture(),
        new TypeResolver()
            .where(
                new TypeCapture<Map<Integer, T>>() {}.capture(),
                new TypeCapture<Map<?, String>>() {}.capture())
            .resolveType(subtype));
  }

  public abstract Iterable<T> children(T root);

  /**
   * Returns an unmodifiable iterable over the nodes in a tree structure, using pre-order traversal.
   * That is, each node's subtrees are traversed after the node itself is returned.
   *
   * <p>No guarantees are made about the behavior of the traversal when nodes change while iteration

 *   <li>All events for a given listener dispatch on the provided {@link #executor}.
 *   <li>It is easy for the user to ensure that listeners are never invoked while holding locks.
 * </ul>
 *
 * The last point is subtle. Often the observable object will be managing its own internal state
 * using a lock, however it is dangerous to dispatch listeners while holding a lock because they