protected Set<E> delegate() {
    validate();
    return delegate;
  }

  private InvalidatableSet(
      Set<E> delegate, Supplier<Boolean> validator, Supplier<String> errorMessage) {
    this.delegate = delegate;
    this.validator = validator;
    this.errorMessage = errorMessage;
  }

  // Override hashCode() to access delegate directly (so that it doesn't trigger the validate() call