* multimap and its {@link Multimap#asMap()} view are {@link ImmutableSortedSet} instances.
     * However, serialization does not preserve that property, though it does maintain the key and
     * value ordering.
     *
     * @since 8.0
     */
    // TODO: Make serialization behavior consistent.
    @CanIgnoreReturnValue
    @Override

    checkNotNull(runnable, "Runnable was null.");
    checkNotNull(executor, "Executor was null.");

    // Lock while we check state. We must maintain the lock while adding the new pair so that
    // another thread can't run the list out from under us. We only add to the list if we have not
    // yet started execution.
    synchronized (this) {
      if (!executed) {

  abstract static class Segment<
          K, V, E extends InternalEntry<K, V, E>, S extends Segment<K, V, E, S>>
      extends ReentrantLock {

    /*
     * Segments maintain a table of entry lists that are ALWAYS kept in a consistent state, so can
     * be read without locking. Next fields of nodes are immutable (final). All list additions are