4. Several exceptions to the compulsory counterclaim rule are set out in the text of Rule 13

(a) itself. In UNION PAVING CO. v. DOWNER CORP.,672276 F.2d 468, 470 (9th Cir.

1960), the court discussed the “pending action” language:

* * * The purpose of this exception is seemingly to prevent one party from compelling another to try his cause of action in a court not of the latter’s choosing when the same cause of action is already the subject of pending litigation in another forum, one which was probably chosen by the owner of the cause of action concerned. * * *