concurring.
I join the judgment of the court. The contract is not unambiguous and, consequently, further proceedings in the district court are necessary. Having reached that conclusion, it is appropriate, of course, to refrain from any further elaboration with respect to the merits.
In my view, we should also refrain from addressing those issues that, because of our disposition, are unnecessary to our decision. In some cases, the inevitability of the recurrence of the issue and the certainty of the proper application of legal principles may justify an appellate court’s proceeding béyond the issues necessary to its decision. In this case, however, where it is quite uncertain that the issues will arise again and where resolution requires that, at least in some instances, we. address complex matters of state law, prudence requires that we refrain from making pronouncements about future litigation whose contours we can only vaguely perceive. This is especially true when, as here, those issues have been addressed by the parties in rather perfunctory fashion.
*1526It is especially important that we observe such self-restraint with respect to the matter of the currency conversion rate. While Congress may well have the authority to resolve this issue, there can be no question that a federal court, exercising jurisdiction on the basis of diversity of citizenship, is bound to apply the law of the state in which it sits. See Klaxon Co. v. Stentor Electric Mfg. Co., 313 U.S. 487, 61 S.Ct. 1020, 85 L.Ed. 1477 (1941). Illinois has not indicated whether, in a case where the substantive law of another state governs the rights and liabilities of the parties, it would consider the matter of the conversion rate one of substance (and therefore governed by the law of that other state) or one of procedure (and therefore governed by the law of Illinois). This substance-procedure dichotomy in choice of law is an elusive issue. See Restatement (Second) of Conflict of Laws § 122 comment a (1971). States have not followed a uniform path through its labyrinth. See, e.g., Henry v. Richardson-Merrell, Inc., 508 F.2d 28 (3d Cir.1975); Heavner v. Uniroyal, Inc., 63 N.J. 130, 305 A.2d 412 (1973) (applying the statute of limitations of the state whose substantive law applied). It is certainly not a matter simply governed by party choice but, as our colleagues in the Second Circuit recognized in Newmont Mines Ltd. v. Hanover Insurance Co., 784 F.2d 127, 138 (2d Cir.1986), and Vishipco Line v. Chase Manhattan Bank, N.A., 660 F.2d 854, 865-67 (2d Cir.1981), cert. denied, 459 U.S. 976, 103 S.Ct. 313, 74 L.Ed.2d 291 (1982), involves a significant public policy determination that, in our federal system, states have the right to make. Its resolution ought not be suggested gratuitously — and therefore to a large extent practically controlled — by a federal court of appeals. Restraint is especially indicated when the suggested analogy to Illinois’ choice of law rule with respect to prejudgment interest, while helpful, is hardly compelling.
The currency conversion issue is an important one and the increase in international commercial litigation has brought it increased recognition. Uniformity of approach will necessarily be achieved. To date, however, Illinois has not spoken on the matter and, apparently, only one federal district judge in Illinois has offered a view with respect to the matter. See Factofrance Heller v. I.P.M. Precision Mach. Co., 627 F.Supp. 1412, 1418-19 (N.D.Ill.1986) (Shadur, J.); Newman-Green, Inc. v. Alfonzo-Larrain R., 612 F.Supp. 1434, 1440-41 (N.D.Ill.1985) (Shadur, J.), vacated on other grounds, 854 F.2d 916 (7th Cir.1988) (en banc), rev’d, — U.S. -, 109 S.Ct. 2218, 104 L.Ed.2d 893 (1989). In the related area of the recognition and enforcement of foreign judgments, we have assumed, in Ingersoll Milling Machine Co. v. Granger, 833 F.2d 680, 692 (7th Cir.1987), that, in the absence of any authority to the contrary, Illinois would follow the Restatement (Second) of Conflict of Laws approach and apply the exchange rate on the date the judgment was recognized and enforced. See Restatement (Second) of Conflict of Laws § 144 comment g (1971). It may be that necessity will once again require the federal judiciary to take its best guess as to how Illinois would handle a specific problem in this area. See Bernhardt v. Polygraphic Co., 350 U.S. 198, 204-05, 76 S.Ct. 273, 277, 100 L.Ed. 199 (1956). However, until that necessity arises, it is best, in my view, that we refrain from setting a course that Illinois, or perhaps Congress, has the right to chart.