system: You are a legal assistant. Answer concisely and accurately. When legal text is provided, rely on it and do not invent facts.

user: Read the following U.S. Supreme Court case excerpt and classify it into the correct legal issue area. Excerpt: 340 U.S. 416 71 S.Ct. 373 95 L.Ed. 389 AMALGAMATED ASS'N OF STREET, ELECTRIC RAILWAY & MOTOR COACH EMPLOYEES OF AMERICA, DIVISION 998, et al.v.WISCONSIN EMPLOYMENT RELATIONS BOARD. No. 330. Argued Jan. 9, 10, 1951. Decided Feb. 26, 1951. Mr. David Previant, Milwaukee, Wis., for petitioner. Mr. J. Gilbert Hardgrove, Milwaukee, Wis., for appellee Milwaukee Gas Light Co. Mr. Martin R. Paulsen, Milwaukee, for respondent Milwaukee Elec. Ry. Transport Co. Mr. Malcolm Riley, Eau Claire, Wis., and Beatrice Lampert, Madison, Wis., for Wisconsin Employment Relations Board. Mr. Chief Justice VINSON delivered the opinion of the Court. 1 The parties to this case are the same transit workers, the same transit company, and the Wisconsin Employment Relations Board before the Court in No. 329, 340 U.S. 383, 71 S.Ct. 359. This action arises out of the same threatened strike discussed in that case. After a restraining order had led to postponement of the strike, the Wisconsin Board appointed arbitrators to 'hear and determine' the dispute in accordance with the terms of the Wisconsin Public Utility Anti-Strike Law. Wis.Stat.1947, § 111.55. Upon the filing of the arbitrators' award, petitioners filed an action in a state circuit court to review that award. Id., § 111.60. That court affirmed the award and the Wisconsin Supreme Court affirmed, 1950, 257 Wis. 53, 42 N.W.2d 477. We granted certiorari in this case together with No. 329, 1950, 340 U.S. 874, 71 S.Ct. 124. 2 In the courts below and in this Court, petitioners attack the arbitration award on the same grounds urged against the Wisconsin Act as a whole in No. 329, and, in addition, raise issues peculiar to the arbitration phase of that act. But we do not reach these issues since it is clear that this case has become moot.1 3 The arbitration award became effective on April 11, 1949. Under the Wisconsin Act, that award 'shall continue effective for one year from that date,' unless sooner terminated by agreement of the parties. Wis.Stat.1949, § 111.59. We are informed that this award was superseded by agreement, and, in any event, the one-year period has elapsed. There being no subject matter upon which the judgment of this Court can operate, the cause is moot. 4 It is argued that the Wisconsin courts have adopted a practice of deciding questions of importance even though the case has become moot, and we are urged to follow that same practice. But whatever the practice in Wisconsin courts, 'A federal court is without power to decide moot questions or to give advisory opinions which cannot affect the rights of the litigants in the case before it. United States v. Alaska S.S. Co., 253 U.S. 113, 115, 116, 40 S.Ct. 448 (449), 64 L.Ed. 808, and cases cited; United States v. Hamburg-Amerikanische Packet-Fahrt-Actien Gesellschaft, 239 U.S. 466, 475—477, 36 S.Ct. 212, 216, 60 L.Ed. 387.' St. Pierre v. United States, 1943, 319 U.S. 41, 42, 63 S.Ct. 910, 911, 87 L.Ed. 1199. 5 It appearing that the cause has become moot, the judgment of the Supreme Court of Wisconsin is vacated without costs and the cause is remanded for such proceedings as by that court may be deemed appropriate. 6 It is so ordered. 7 Judgment vacated and cause remanded. 1 It has also been argued that No. 329 and No. 438 are moot by reason of the settlement of the immediate dispute which led to the strike action in each case. The injunction before us in No. 329 is 'perpetual' by its terms so that the action does not become moot even though the decree be obeyed. J. I. Case Co. v. National Labor Board, 1944, 321 U.S. 332, 334, 64 S.Ct. 576, 578, 88 L.Ed. 762; Federal Trade Comm. v. Goodyear Tire & Rubber Co., 1938, 304 U.S. 257, 260, 58 S.Ct. 863, 864, 82 L.Ed. 1326, and cases cited therein. As to No. 438, the judgment below imposes fines upon petitioners. No question of mootness can be raised so long as enforcement of that judgment is sought.

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