A pension fund imposed withdrawal liability upon a company that had withdrawn from a multiemployer pension plan in which its union employees had participated. The company demanded indemnification from the union pursuant to the collective bargaining agreement ... [An] arbitrator ordered the union to pay.... The Sixth Circuit affirmed, rejecting an argument that it would violate public policy for a union to indemnify an employer for any contingent liability to a pension plan established under [ERISA.]"