Opinion for the Court filed by Circuit Judge WALD.
Kaiser Steel Corporation (“Kaiser”), a coal mine operator, obtained a number of indemnity bonds in order to fulfill its self-insurance responsibilities under the Black Lung Benefits Act (“the Act”), 30 U.S.C. §§ 901-945 (1994), which imposes liability on mine operators for payment of benefits to miners who have developed pneumoconiosis (“black lung”). One of the bonds, issued in 1982 and canceled in 1984, named Insurance Company of North America (“INA”) as the surety. When Kaiser filed for bankruptcy in 1987, INA, as Kaiser’s surety, was obligated to pay covered claims under the bond, a fact INA has never challenged. What has remained in dispute, however, is how to determine the claims for which INA is liable. In its first appearance before this court,
United States v. Insurance Co. of N. Am.,
I. BACKGROUND
Under the Act, coal miners who have become totally disabled due to black lung that arises at least in part out of their coal mine employment and certain surviving dependents of miners whose death was due to black lung are entitled to monthly benefits. As of January 1, 1974, claims for benefits must be filed pursuant to the applicable state workers’ compensation law if that law has been deemed by the Secretary of Labor to provide adequate coverage for black lung. 30 U.S.C. § 931 (1994); 20 C.F.R. § 722.101 (1997). Where no approved state workers’ *1039 compensation statute exists, claims are filed with the Secretary of Labor and are paid by responsible coal mine operators. 30 U.S.C. § 932(b) (1994); 20 C.F.R. § 725.1(d) (1997). An operator is considered a “responsible operator” under the Department of Labor’s regulations if it is the operator “with which the miner had the most recent periods of cumulative employment of not less than 1 year.” 20 C.F.R. § 725.493(a)(1) (1997). This regulation ensures that only one mine operator is responsible for the payment of a particular miner’s benefits. 2
The Act requires that each operator secure the payment of benefits for which it is liable in advance, either by qualifying as a self-insurer pursuant to 20 C.F.R. §§ 726.101 et seq. or by obtaining outside insurance. 30 U.S.C. § 933(a) (1994); 20 C.F.R. § 725.494 (1997). A mine operator that self-insures must acquire either an indemnity bond or negotiable securities in an amount sufficient to discharge its liability under the Act. 20 C.F.R. § 726.101 (1997). If a responsible operator or its surety does not make timely payments, or if no operator is liable for payment, benefits are paid from the Black Lung Disability Trust Fund (“the Fund”), which is financed by a tax on most types of coal and for which the Secretary of the Treasury, the Secretary of Labor, and the Secretaiy of Health and Human Services are trustees. 30 U.S.C. § 934 (1994); 26 U.S.C. § 9501 (1994); 20 C.F.R. § 725.1(g) (1997). A responsible operator is then liable to the federal government for repayment to the Fund of any expenditures attributable to that operator. 30 U.S.C. § 934; 20 C.F.R. § 725.603 (1997).
In 1973 the Department of Labor (“the Department”) authorized Kaiser to act as a self-insured coal mine operator. To fulfill its obligations under section 726.101, Kaiser obtained two indemnity bonds from INA, one in the amount of $684,750 (effective November 2, 1973) and one in the amount of $3,304,000 (effective May 1, 1982, and canceled on May 20, 1984) (“the 1982 bond”). Kaiser filed for bankruptcy in February 1987 and stopped paying benefits to its miners. In June 1987, the Secretary of Labor wrote to INA, requesting that INA, as Kaiser’s surety, make arrangements for payment under the 1982 bond for claims filed from July 1, 1973, to May 20, 1984. Relying on the language of the bond, which defined INA’s liability as that “which attaches to or is accrued by” Kaiser during the bond period, INA declined to pay all but two claims, asserting that the remainder of the claims accrued before 1982. The Department arranged for payment of the outstanding claims from the Fund and continued to request payment from INA. When its efforts proved unsuccessful, the Department filed suit in the district court on December 30,1993.
Both parties moved for summary judgment as to the scope of INA’s liability.
3
The district court entered judgment against INA on October 12, 1994, agreeing with the government that language in the bond that rendered INA liable for all of Kaiser’s “present, past, and potential liability” under the Act created a broad scope of liability “for all Kaiser’s obligations existing as of the effective date and continuing until the termination of the bond.”
United States v. Insurance Co. of N. Am.,
*1040 On December 9, 1994, the government moved for leave to file, in support of its proposed judgment, a declaration by Scott D. Valentine (“Valentine”), a computer specialist with the Depart’ rent who had reviewed the Department’s records and calculated the amount due from INA. INA objected to the Valentine declaration, asserting that Valentine had not previously been identified as a witness and that the information upon which the declaration was based had not been disclosed during discovery. INA asked that its objections be sustained or, in the alternative, that it be granted 20 days in which to verify the accuracy of Valentine’s calculations. The district court granted both the government’s motion for leave to file the declaration and INA’s request for review time on January 17, 1995, and on March 24, 1995, it entered judgment against INA in the amount of $659,871.80, plus interest, and ordered INA liable for payment of all future benefits arising from employment with Kaiser on or before May 20, 1984, capped by the penal sum of the bond.
INA appealed the district court’s judgment, arguing that the district court had misinterpreted the bond language and thus incorrectly broadened the scope of its liability to include claims in existence before the bond period began.
5
We rejected the district court’s interpretation, holding that because the parties intended that the bond impose on INA “only liability accruing after the bond’s effective date,” INA would be liable only for claims for which Kaiser became the “responsible operator” under 20 C.F.R. § 725.493 between May 1, 1982, and May 20, 1984, the date the bond was canceled, “by virtue of an employee’s completion of one full year of employment with Kaiser during that period.”
INA I,
On remand, the government moved for entry of judgment in the amount of $350,-604.49, which represented the amounts due on claims related to three miners, each of whom had concluded his employment with Kaiser in 1983. 6 In support of this motion, the government submitted a new declaration from Valentine, which, like the first declaration, described how the amount due had been calculated; this amount included payments made from the Fund while the case had been on appeal. INA opposed the government’s motion, arguing that the limitation in our opinion — the requirement of “an employee’s completion of one full year of employment with Kaiser during [the bond] period” — restricted INA’s liability to those claims filed by miners whose first year of employment with Kaiser was completed during the bond period, not the last year of employment, as the government contended. INA also renewed its objections to Valentine’s declaration, asserting that it contained hearsay in its references to the Department’s computer databases, that INA had not had the opportunity to depose Valentine, and that it contained inadmissible legal conclusions regarding INA’s liability.
On January 22, 1997, the district court entered judgment in favor of the government in the amount of $350,604.49, plus interest, and ordered INA to pay any future benefits that became due to individuals whose last mining employment of at least one year was with Kaiser and ended during the bond period. It rejected INA’s argument that its liability was determined by a miner’s first year of employment as “contrary to the plain text of the regulations, counter to the Court of Appeals’ decision, and entirely without merit,” United States v. Insurance Co. of N. Am., No. 93-2660, slip op. at 3-4 (D.D.C. Jan. 22, 1997), and declined to entertain INA’s challenge to the Valentine declaration because INA had not raised the issue on appeal. INA now appeals a second time.
II. Analysis
Because both parties have framed the debate as one regarding the interpreta
*1041
tion of our previous opinion, the primary question before us is whether that opinion dictated the conclusion reached by the district court. As we have previously noted, the “mandatq rale,” an application of the “law of the case” doctrine, states that a district court is bound by the mandate of a federal appellate court and generally may not reconsider issues decided on a previous appeal.
See, e.g., Maggard v. O’Connell,
At issue before us on the first appeal in this case was a narrow question regarding the scope of INA’s liability: whether INA was hable for ah claims
outstanding
during the bond period or only for those claims
arising
during the bond period. The government had claimed, and the district court had agreed, that the bond’s “broad” language and references to “present, past, and potential liability” led to the conclusion that INA would be hable for any claim that was outstanding between 1982 and 1984, even if the condition that made INA liable for that claim — that Kaiser had become the “responsible operator” — had been fulfilled before the first day of the bond period. We disagreed, holding that such an interpretation would “virtually delete[ ]” the penultimate sentence of the .second clause of the bond, which defined INA’s liability as that “which attaches to or is accrued by [Kaiser] in or for the period during which this bond is in force.”
INA I,
The district court on remand, however, read this holding more broadly, believing it not only to require such a temporal connection but also to define how that connection should be determined. The parties now before us commit the same error, each contending that our opinion dictates the year of employment that fulfills the one-year requirement and each mustering various regulatory language and policy rationales in its favor. But these arguments are misdirected: As we have noted above, a careful reading of the opinion reveals that the issue was not decided, nor was any resolution necessarily implied by our holding. Simply put, we did not decide which year of employment the parties to the bond intended to trigger INA’s liability.
If the answer to this question were clear from the language of the bond, it would be incumbent upon us to decide it on this appeal. The indemnity bond at issue in this
*1042
case is a contract between the principal (here, Kaiser) and the surety (INA) in favor of an obligee (the U.S. government); as with any contract, if its terms are unambiguous on their face, interpretation is considered a question of law appropriately resolved by this court.
See, e.g., NRM Corp. v. Hercules, Inc.,
We thus return to the bond provision we examined in the first appeal to determine if there is an unambiguous reference to the year of employment that should serve as the trigger of liability, keeping in mind that a contract provision is ambiguous “if it is reasonably susceptible of different constructions, but it is not ambiguous merely because the parties later disagree on its meaning.”
Bennett Enters., Inc. v. Domino’s Pizza, Inc.,
When a contract incorporates a regulation by reference, that regulation becomes a part of the contract for the indicated purposes as if the words of that regulation were set out in full in the contract.
See, e.g., Washington Metro. Area Transit Auth. v. Mergentime Corp.,
Our decision to remand once again is entirely consistent with our action in the first appeal, in which we resolved the interpretation issue before us at that time. There, the provisions we were required to reconcile— the reference to “present, past, and potential liability” and the reference to liability as that which “attaches to or is accrued by” Kaiser during the bond period — were each unambiguous on their face with respect to the conflicting claims then presented. Our task, appropriately undertaken under the rules of contract interpretation, was to harmonize these provisions into a consistent whole. Here, by contrast, we are faced with a single provision, the meaning of which cannot be unlocked with the interpretive keys we, as an appellate court, possess. It would be appropriate for us to resolve this ambiguity only if “the proper resolution was so obvious on the basis of the record that a remand on that issue would [be] ‘unduly wasteful of judicial resources.’ ”
Maggard,
III. Conclusion
We vacate the district court’s judgment and order and remand to allow the district court to determine the parties’ intent with respect to the year of employment that triggers Kaiser’s — and therefore INA’s — liability for claims under the Act. 12
It is so ordered.
Notes
. Because our conclusion necessarily implies that INA’s appeal was not frivolous, we deny the government's motion for sanctions.
. To be precise, a finding that an operator is a "responsible operator” creates a rebuttable presumption that a miner's black lung arose out of his employment with that operator. The operator is therefore liable for payment of benefits to that miner unless it successfully rebuts that presumption. 20 C.F.R. § 725.493(a)(6) (1997).
. INA also moved for summary judgment on the ground that the government's claim was barred by the statute of limitations in 28 U.S.C. § 2415(a) (1994). That assertion was rejected both by the district court and by this court on appeal,
see INA I,
. The only coverage issue before the district court was the scope of the 1982 bond; although the second count of the government's complaint sought reimbursement under the 1973 bond, the government moved for, and was granted, summary judgment only as to the 1982 bond.
. INA did not raise any challenge to the Valentine declaration on appeal, although the declaration was listed as a subject of appeal in INA's Statement of Issues to Be Raised.
. The three claimants were Lucas S. Marez, whose last employment date was September 6, 1983; Arthur R. Mena, whose last employment date was January 14, 1983; and Ida Miller, widow of Leroy F. Miller, whose last employment date was June 24, 1983.
. As we have previously noted, it is entirely appropriate — and, in most cases in this circuit, necessary — to consult the opinion to interpret the mandate.
See, e.g., City of Cleveland,
. Subject matter jurisdiction in this case was obtained under,
inter alia,
28 U.S.C. § 1352 (1994), which mandates original jurisdiction in the district courts, concurrent with state courts, "of any action on a bond executed under any law of the United States, except matters within the jurisdiction of the Court of International Trade under section 1582 of this title." Because "[tjhere is no federal legislative standard for bonds 'executed under any law of the United States,’ ”
Skirlick v. Fidelity & Deposit Co. of Md.,
. We are mindful of the fact that the bond states that liability attaches "as is determined by the Secretary of Labor pursuant to said Act of 1969 and the applicable regulations duly promulgated thereunder,” J.A. 12 (emphasis added), language that could support an interpretation that discretion to determine liability is wholly vested with the Secretary of Labor. Nevertheless, we are confident, given that the bond refers to a determination "pursuant to” the Act and the regulations, that the parties did not intend that the Secretary’s discretion would be unfettered. Except for rejecting an interpretation vesting total discretion in the Secretary, we take no position on the circumstances, if any, under which a Departmental interpretation of the regulations would be entitled to deference.
. The current version of 20 C.F.R. § 725.493 was promulgated on August 18, 1978, and so was in existence when the bond at issue in this case went into effect. See 43 Fed.Reg. 36,771, 36,804 (1978).
. The canon of construction known as
contra proferentum
— that ambiguities in an insurance contract should be construed against the insurer who drafted the contract,
see, e.g., Revere Copper & Brass, Inc. v. Overseas Private Inv. Corp.,
. Because the district court on remand is permitted to admit extrinsic evidence to aid it in resolving the bond's ambiguity, INA should be permitted to challenge the new Valentine declaration submitted by the government in support of its claim. Although we stated in
Crocker v. Piedmont Aviation, Inc.,
