"Not only may the power to tax be exercised oppressively, but for
one government--state or national--to lay a tax on the instrumentalities
or securities of the other is derogatory to the latter's dignity, subversive
of its powers, and repugnant to its paramount authority. See California
Central P.R. Co., 127 U.S. 1, 41, 8 S.Ct. 1073 (32 L.Ed. 150). These constitute special and compelling reasons why courts, in scrutinizing
taxing acts like that here involved, should be acute to distinguish
between an exaction which is substance and reality is what it pretends
to be, and a scheme to lay a tax upon a nontaxable subject by a deceptive
use of words."
[Macallen Co. v. Commonwealth of Massachusetts, 279 U.S. 620, 49 S.Ct. 432 (1929)]