2 JAM v. INTERNATIONAL FINANCE CORP. Syllabus continuously equivalent. The IOIA is thus like other statutes that use similar or identical language to place two groups on equal foot- ing. See, e.g., Civil Rights Act of 1866, 42 U.S. C. §§1981(a), 1982; Federal Tort Claims Act, 28 U.S. C. §2674. Whatever the ultimate purpose of international organization immunity may be, the immedi- ate purpose of the JOIA immunity provision is expressed in language that Congress typically uses to make one thing continuously equiva- lent to another. Pp. 6-9. (b) That reading is confirmed by the “reference canon” of statutory interpretation. When a statute refers to a general subject, the stat- ute adopts the law on that subject as it exists whenever a question under the statute arises. In contrast, when a statute refers to anoth- er statute by specific title, the referenced statute is adopted as it ex- isted when the referring statute was enacted, without any subse- quent amendments. Federal courts have often relied on the reference canon to harmonize a statute with an external body of law that the statute refers to generally. The IOJA’s reference to the immunity en- joyed by foreign governments is to an external body of potentially evolving law, not to a specific provision of another statute. Nor is ita specific reference to a common law concept with a fixed meaning. The phrase “immunity enjoyed by foreign governments” is not a term of art with substantive content but rather a concept that can be given scope and content only by reference to the rules governing foreign sovereign immunity. Pp. 9-11. (c) The D. C. Circuit relied upon Atkinson’s conclusion that the ref- erence canon’s probative force was outweighed by an IOJA provision authorizing the President to alter the immunity of an international organization. But the fact that the President has power to modify otherwise applicable immunity rules is perfectly compatible with the notion that those rules might themselves change ov