* Returns a serializable form of this object. Non-public subclasses should not override this
   * method. Publicly-accessible subclasses must override this method and should return a subclass
   * of SerializedForm whose readResolve() method returns objects of the subclass type.
   */
  @J2ktIncompatible // serialization
  Object writeReplace() {
    return new SerializedForm<>(this);
  }

   *
   * @since 2.0
   */
  public static class Builder<E> extends ImmutableCollection.Builder<E> {
    /*
     * `contents` is null only for instances of the subclass, ImmutableSortedMultiset.Builder. That
     * subclass overrides all the methods that access it here. Thus, all the methods here can safely
     * assume that this field is non-null.
     */
    @CheckForNull ObjectCountHashMap<E> contents;

   */
  public static <K extends @Nullable Object, V extends @Nullable Object> SortedMap<K, V> filterKeys(
      SortedMap<K, V> unfiltered, final Predicate<? super K> keyPredicate) {
    // TODO(lowasser): Return a subclass of Maps.FilteredKeyMap for slightly better
    // performance.
    return filterEntries(unfiltered, Maps.<K>keyPredicateOnEntries(keyPredicate));
  }

  /**