this.delegate = checkNotNull(delegate);
      this.mutex = (mutex == null) ? this : mutex;
    }

    Object delegate() {
      return delegate;
    }

    // No equals and hashCode; see ForwardingObject for details.

    @Override
    public String toString() {
      synchronized (mutex) {
        return delegate.toString();
      }
    }

    // Serialization invokes writeObject only when it's private.