private int remaining() {
    requireNonNull(seq); // safe as long as we call this only after checkOpen
    return seq.length() - pos;
  }

  /*
   * To avoid the need to call requireNonNull so much, we could consider more clever approaches,
   * such as:
   *
   * - Make checkOpen return the non-null `seq`. Then callers can assign that to a local variable or

The `TestClient` is based on [HTTPX](https://www.python-httpx.org), and luckily, we can use it directly to test the API.

## Example { #example }

For a simple example, let's consider a file structure similar to the one described in [Bigger Applications](../tutorial/bigger-applications.md) and [Testing](../tutorial/testing.md):

```
.
├── app
│   ├── __init__.py
│   ├── main.py

 * <ul>
 *   <li>Call {@link #clearCredentials()} after authentication is complete to clear sensitive data from memory</li>
 *   <li>Avoid logging instances of this class as it may expose credentials</li>
 *   <li>Consider using external secret management systems for credential storage in production</li>
 * </ul>
 *
 * @author shinsuke
 */
public class FtpAuthentication {

   *     such index exists.
   */
  public static int indexOf(byte[] array, byte target) {
    return indexOf(array, target, 0, array.length);
  }

  // TODO(kevinb): consider making this public
  private static int indexOf(byte[] array, byte target, int start, int end) {
    for (int i = start; i < end; i++) {
      if (array[i] == target) {
        return i;
      }
    }

  if (gradient_function == nullptr) {
    return absl::InvalidArgumentError(absl::StrCat(
        "Provided null gradient_function for '", op_type, "'.\n",
        "If the intent is to treat this op as non-differentiable consider ",
        "using RegisterNotDifferentiable or ",
        "NotDifferentiableGradientFunction."));
  }
  return gradient_function->Compute(ctx_, output_gradients, result);
}

 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}
 *       even on an {@code ImmutableMap<K, @NonNull String>}.
 * </ul>
 *
 * <p>Consumers of this annotation include:
 *
 * <ul>
 *   <li>NullAway, which <a
 *       href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treats it

 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}
 *       even on an {@code ImmutableMap<K, @NonNull String>}.
 * </ul>
 *
 * <p>Consumers of this annotation include:
 *
 * <ul>
 *   <li>NullAway, which <a
 *       href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treats it

 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}
 *       even on an {@code ImmutableMap<K, @NonNull String>}.
 * </ul>
 *
 * <p>Consumers of this annotation include:
 *
 * <ul>
 *   <li>NullAway, which <a
 *       href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treats it

    @ParametricNullness
    V leftValue();

    /** Returns the value from the right map (possibly null). */
    @ParametricNullness
    V rightValue();

    /**
     * Two instances are considered equal if their {@link #leftValue()} values are equal and their
     * {@link #rightValue()} values are also equal.
     */
    @Override
    boolean equals(@Nullable Object other);

    /**

 *       ImmutableMap.get} returns {@code @Nullable E} because the method can return {@code null}
 *       even on an {@code ImmutableMap<K, @NonNull String>}.
 * </ul>
 *
 * <p>Consumers of this annotation include:
 *
 * <ul>
 *   <li>NullAway, which <a
 *       href="https://github.com/google/guava/issues/6126#issuecomment-1204399671">treats it