}

    @VisibleForTesting
    void forceJdk() {
      requireNonNull(impl); // see the comment on the field
      this.impl = new JdkBackedSetBuilderImpl<>(impl);
    }

    final void copyIfNecessary() {
      if (forceCopy) {
        copy();
        forceCopy = false;
      }
    }

    void copy() {
      requireNonNull(impl); // see the comment on the field
      impl = impl.copy();