1. Rule 13

(a) is silent on the effect of failing to plead a compulsory counterclaim. It seems clear that an unasserted compulsory counterclaim cannot be raised in a subsequent suit in a federal court, see, e.g., Twin Disc, Inc. v. Lowell, 69 F.R.D. 64 (E.D. Wis. 1975), although courts differ as to whether this conclusion follows from principles of res judicata, waiver, or estoppel. See Scott,Collateral Estoppel by Judgment, 56 Harv. L. Rev. 1 (1942) (res judicata); Wright, Estoppel by Rule: The Compulsory Counterclaim Under Modern Pleading, 38 Minn. L. Rev. 423 (1954) (estoppel). Under the Minnesota rule discussed in Note 4, p. 670, supra, would any of these theories bar a defendant from asserting a transactionally related tort claim in an independent action rather than as a counterclaim?