Under the “Auer doctrine,” named for the 1997 decision Auer v. Robbins, courts accept an agency’s interpretation of its own ambiguous regulation unless that interpretation is clearly erroneous, or flatly inconsistent with the text of the regulation, or unreasonable, or something like that. Auer is a principle of long standing. Just how long is one of the sources of disagreement in Kisor v. Wilkie, but however you count, it is a doctrine universally understood as well-settled until relatively recently. But a revolt has been brewing.