{9} Allstate Insurance immediately reacted to this decision. See, e.g., Allstate Ins. Co. v. Indep. Appliance Refrigeration Serv., Inc., 278 F.3d 1102, 1104 (10th Cir. 2002) ("In 1990, Allstate decided to end the practice of `stacking'. . . . Allstate amended the policy in 1997 . . . to allow stacking of two `but no more than two' coverages."); Allstate Ins. Co. v. Stone, 116 N.M. 464, 465, 863 P.2d 1085, 1086 (1993) ("Until April 1990, Allstate [Insurance] had charged separate premiums. . . . After April 1990, however, Allstate charged one premium . . . on their multi-car policy.").

Rule 12-607(A)(1). Federal courts have the option of determining what a state supreme court would do if confronted with the same issue, see Erie R.R. Co. v. Tompkins, 304 U.S. 64, 78 (1938), or of certifying the question to the state supreme court for determination, see Lehman Bros. v. Schein, 416 U.S. 386, 390-91 (1974) ("The decision to certify a question to the state supreme court 'rests in the sound discretion of the federal court.'");Farm Bureau Mut. Ins. Co. v. Jameson, 472 F. Supp. 2d 1272, 1280 (D.N.M. 2006) (Browning, J.); Allstate v. Stone, 116 N.M. 464, 465, 863 P.2d 1085, 1086 (1993) ("This matter comes before us by way of certification from the United States District Court for the District of New Mexico."). Pursuant to N.M.S.A. 1978, § 39-7-4, the Supreme Court of New Mexico may answer questions that the federal district court certifies to it "if the answer may be determinative of an issue in pending litigation in the certifying court and there is no controlling appellate decision, constitutional provision or statute of this state." N.M.S.A. 1978, § 39-7-4.

Despite the strength of the judicial policy . . . as discussed in, e.g., Jimenez, 107 N.M. at 324-26, 757 P.2d at 794-96, it may be possible to give effect to a truly unambiguous antistacking clause, provided it plainly notifies the insured that only one premium has been charged for one insurance coverage, that the coverage provides personal accident insurance that cannot be stacked regardless of the number of vehicles covered by the policy, and that the insured should bear this feature in mind when purchasing insurance. As we explained in Allstate Ins. Co. v. Stone, 116 N.M. 464, 863 P.2d 1085 (1993), concerning the "repugnancy" of irreconcilable exclusionary language, "when 'an exclusionary clause simply nullifies the grant' of coverage, this Court will 'refuse to apply the clause that deprives the insured of the insurance coverage which the insured reasonably understood was afforded by the policy.' " Id. at 467, 863 P.2d at 1088 (quoting Federal Ins. Co. v. Century Fed. Sav. Loan Ass'n., 113 N.M. 162, 169, 824 P.2d 302, 309 (1992)).

See, e.g., id. at 171, 646 P.2d at 1235. In one of our most recent stacking cases, Allstate Insurance Co. v. Stone, 116 N.M. 464, 863 P.2d 1085 (1993), we considered a multi-car policy in which the insurer charged a single premium for coverage with respect to all vehicles insured by the policy. We held that stacking of the coverages was permitted, but did not reach the effect of the single-premium charge.

{3} In this opinion, we treat UIM and UM policies as equivalents and often use the terms interchangeably. See Allstate Ins. Co. v. Stone, 116 N.M. 464, 465 n. 1, 863 P.2d 1085, 1086 n. 1 (1993) (noting that UIM polices are equivalent to UM policies for purposes of stacking); see also NMSA 1978, § 66-5-301(B) (1983) (governing UM coverage and expressly including UIM coverage for those protected by an insured's policy). In deciding this case, we follow the trend in our case law that treats UIM and UM claims as contract-based actions, and we join the majority view in holding that the statute of limitations on a UIM claim begins to run upon breach of the insurance contract, where neither the UM statute nor the insurance policy provide otherwise.

20. Plaintiff is correct that, under New Mexico law, a policy exclusion is repugnant to the insuring clause when, rather than setting out a specific exception to a broad grant of coverage, the exclusionary clause simply nullifies the grant. Federal Ins. Co. v. Century Fed. Sav. Loan Ass'n, 113 N.M. 162, 169, 824 P.2d 302, 309 (1992); see also Allstate Ins. Co. v. Stone, 116 N.M. 464, 467, 863 P.2d 1085, 1088 (1993); Western Heritage Ins. Co. v. Chava Trucking, Inc., 991 F.2d 651, 655-56 (10th Cir. 1993). Where there is a repugnancy of provisions, the court will not enforce a clause that deprives an insured of coverage which the insured may reasonably have believed was afforded under the policy.

Uninsured motorist coverage includes underinsured motorist coverage. Allstate Ins. Co. v. Stone, 116 N.M. 464, 465 n. 1, 863 P.2d 1085, 1086 n. 1 (1993); see NMSA 1978, § 66-5-301(B) (Repl.Pamp. 1994).

Allstate's attempt to preclude stacking entirely was voided by the New Mexico Supreme Court in Allstate Insurance Co. v. Stone because of contradictory language in the policy indicating that stacking may be available. 116 N.M. 464, 863 P.2d 1085, 1087-88 (1993). Allstate revised its policy language in 1994 to eliminate the ambiguity and sent endorsement AU2207-1 with "Important Notice" X5045 to its insureds.

See also Walker v. United States, 2007-NMSC-038, ¶ 1, 142 N.M. 45, 162 P.3d 882, P.3d 882, 884 (2007)(answering questions that the United States Court of Federal Claims certified); Campos v. Murray, 2006-NMSC-020, ¶ 2, 139 N.M. 454, 134 P.3d 741, 742 (answering questions that the Honorable Bruce D. Black, United States District Judge for the United States District Court for the District of New Mexico, certified). Federal courts have the option of determining what a state court would do if confronted with the same issue, see Erie Railroad Co. v. Tompkins, 304 U.S. 64, 78, 58 S.Ct. 817, 82 L.Ed. 1188 (1938), or of certifying the question to the state appellate court for review, see Allstate v. Stone, 1993-NMSC-066, ¶ 1, 116 N.M. 464, 863 P.2d 1085, 1086 ("This matter comes before us by way of certification from the United States District Court for the District of New Mexico."). See also Lehman Bros. v. Schein, 416 U.S. 386, 390-91, 94 S.Ct. 1741, 40 L.Ed.2d 215 (1974) ("The decision to certify a question to the state supreme court ‘rests in the sound discretion of the federal court.").

Hartford Ins. Co. of the Midwest v. Cline, 367 F.Supp.2d 1342, 1344 (D.N.M. 2005) (quoting N.M. Stat. Ann. § 39-7-4). “Federal courts have the option of determining what a state court would do if confronted with the same issue, see Erie R.R. Co. v. Tompkins, 304 U.S. 64, 78 (1938), or of certifying the question to the state appellate court for review, see Allstate v. Stone, 863 P.2d 1085, 1086 (N.M. 1993) (‘This matter comes before us by way of certification from the United States District Court for the District of New Mexico.').”