1. Whether a suit seeking to enforce a state-law arbitration obligation brought under Section 4 of the Federal Arbitration Act, 9 U.S.C. § 4, "aris[es] under" federal law, see 28 U.S.C. § 1331, when the petition to compel itself raises no federal question but the dispute sought to be arbitrated--a dispute that the federal court is not asked to and cannot reach-- involves federal law.2. If so, whether a "completely preempted" state-law counterclaim in an underlying state-court dispute can supply subject matter jurisdiction.