* Send the invoice to the client.
    * Collect the money from the client.
    * Send a notification back to the API user (the external developer), as a callback.
        * At this point is that the API will somehow send a POST request to the
            external API with the notification of the invoice event
            (e.g. "payment successful").
    """
    # Send the invoice, collect the money, send the notification (the callback)

    /*
     * Since we use AtomicReferenceFieldUpdaterAtomicHelper by default, we'll "obviously" use it
     * even when Unsafe isn't available. But it's nice to have a check here to make sure that
     * nothing somehow goes wrong as the JDK restricts access to Unsafe.
     */
    checkHelperVersion(NO_UNSAFE, "AtomicReferenceFieldUpdaterAtomicHelper");
    /*

          interrupted = true;
        }
      }
    } finally {
      if (interrupted) {
        Thread.currentThread().interrupt();
      }
    }
  }

  // TODO(user): Support Sleeper somehow (wrapper or interface method)?
  /**
   * Invokes {@code unit.}{@link TimeUnit#sleep(long) sleep(sleepFor)} uninterruptibly.
   *
   * @since 33.4.0 (but since 28.0 in the JRE flavor)
   */
  @J2ktIncompatible

 *     classes can be chained onto them without casting.
 * @param <G> The type of the generator to be passed to testers in the generated test suite. An
 *     instance of G should somehow provide an instance of the class under test, plus any other
 *     information required to parameterize the test.
 * @author George van den Driessche
 */
@GwtIncompatible
public abstract class FeatureSpecificTestSuiteBuilder<

 *     classes can be chained onto them without casting.
 * @param <G> The type of the generator to be passed to testers in the generated test suite. An
 *     instance of G should somehow provide an instance of the class under test, plus any other
 *     information required to parameterize the test.
 * @author George van den Driessche
 */
@GwtIncompatible
public abstract class FeatureSpecificTestSuiteBuilder<

 *   ...
 * }
 * </pre>
 *
 * <p>No cascading checks are performed against the return values of methods unless the method is a
 * static factory method. Neither are semantics of mutation methods such as {@code
 * someList.add(obj)} checked. For more detailed discussion of supported and unsupported cases, see
 * {@link #testEquals}, {@link #testNulls} and {@link #testSerializable}.
 *

* etc.

In these cases, you would normally return an **HTTP status code** in the range of **400** (from 400 to 499).

This is similar to the 200 HTTP status codes (from 200 to 299). Those "200" status codes mean that somehow there was a "success" in the request.

The status codes in the 400 range mean that there was an error from the client.

Remember all those **"404 Not Found"** errors (and jokes)?

      TestCondition testCondition = create();

      ScheduledExecutorService scheduledPool = newScheduledThreadPool(1);
      // If signal() fails somehow, we should see a failed test, even without looking at the Future.
      Future<?> unused = scheduledPool.schedule(testCondition::signal, delay, unit);

      return testCondition;
    }

    -->
    <surefire.toolchain.version>${java.specification.version}</surefire.toolchain.version>
    <!-- Override this with -Dtest.include="**/SomeTest.java" on the CLI -->
    <test.include>%regex[.*.class]</test.include>
    <animal-sniffer-maven-plugin.version>1.27</animal-sniffer-maven-plugin.version>
    <asm.version>9.9.1</asm.version>

 *   ...
 * }
 * </pre>
 *
 * <p>No cascading checks are performed against the return values of methods unless the method is a
 * static factory method. Neither are semantics of mutation methods such as {@code
 * someList.add(obj)} checked. For more detailed discussion of supported and unsupported cases, see
 * {@link #testEquals}, {@link #testNulls} and {@link #testSerializable}.
 *