What is the test for applying federal law to a state claim?

The high court went on to say: "Just as federal courts are constitutionally obligated to apply state law to state claims, [citation], so too the Supremacy Clause imposes on state courts a constitutional duty 'to proceed in such manner that all the substantial rights of the parties under controlling federal law [are] protected.' " (Felder v. Casey, supra, 487 U.S. 131, 151, 108 S.Ct. 2302; see 1 Nahmod, Civil Rights and Civil Liberties Litigation (4th ed.1997) 1.59, pp. 1-91--1-93.)

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