Under federal substantive law, courts must place arbitration agreements on an equal footing with other contracts, and enforce them according to their terms. AT&T Mobility LLC v. Concepcion, 131 S. Ct. 1740, 1745 (2011). By the same token, §2 also permits arbitration agreements to be declared unenforceable "upon such grounds as exist at law or in equity for the revocation of any contract." This saving clause permits agreements to arbitrate to be invalidated by such generally applicable contract defenses as fraud, duress, or unconscionability, but not by defenses that apply only to arbitration or that derive their meaning from the fact that an agreement to arbitrate is at issue. Concepcion, 131 S. Ct. at 1746. Applying this framework, the Supreme Court has held that parties may, for instance, agree to limit the issues subject to arbitration, or limit with whom a party will arbitrate its disputes. Mitsubishi Motors Corp. v. Soler Chrysler-Plymouth, Inc., 473 U.S. 614, 628 (1985).