836
Opinion of the Court
and they explicitly define [CNOL] without an accompanying reference to consolidated [PLEs]. This omission . . . makes clear that blending those expenses is not permitted . . . ." 208 F. 3d, at 458.
We think the omission of PLEs from the series of items that § 1.1502-12 requires to be tallied at the consolidated level has no such clear lesson, however. The logic that invests the omission with significance is familiar: the mention of some implies the exclusion of others not mentioned. Leatherman v. Tarrant County Narcotics Intelligence and Coordination Unit, 507 U. S. 163, 168 (1993) ("Expressio unius est exclusio alterius"). But here, as always, the soundness of that premise is a function of timing: if there was a good reason to consider the treatment of consolidated PLL at the time the regulation was drawn, then omitting PLL from the list of items for consolidated treatment may well have meant something. But if there was no reason to consider PLL then, its omission would mean nothing at all. And in fact there was no reason. When the consolidated return regulations were first promulgated in 1966, there was no carryback provision pegged to PLEs or PLLs; those notions did not become separate carryback items until 1978, when the 10-year rule was devised. See Revenue Act of 1978, § 371, 92 Stat. 2859; see also Leatherman, Current Developments 393, n. 5. Omission of PLEs or PLLs from the series set out for consolidated treatment in the 1966 regulation therefore meant absolutely nothing in 1966. The issue, then, is the significance, not of omission, but of failure to include later: has the significance of the earlier regulation changed solely because the Treasury has never amended it, even though PLL is now a separate carryback? We think that is unlikely. The Treasury's relaxed approach to amending its regulations to track Code changes is well documented. See e. g., Dubroff 41-72, n. 193; Axelrod & Blank 1391; Leatherman, Separate Liability Losses 708-709. The absence of any amendment to § 1.1502-12 that might have added PLEs
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