* cancelled).
 *
 * <p>This class serializes execution of <i>submitted</i> tasks but not any <i>listeners</i> of
 * those tasks.
 *
 * <p>Submitted tasks have a happens-before order as defined in the Java Language Specification.
 * Tasks execute with the same happens-before order that the function calls to {@link #submit} and
 * {@link #submitAsync} that submitted those tasks had.
 *