In United States v. Cook (1872) 84 U.S. 168, the Supreme Court held that where a statute defining an offense contains an exception in its enacting clause, which is so incorporated with the language defining the offense that the ingredients of the offense cannot be accurately and clearly described if the exception is omitted, an indictment founded upon the statute must allege enough to show that the accused is not within the exception; but where the language of the section defining the offense is so entirely separable from the exception that the ingredients constituting the offense may be accurately and clearly defined without reference to the exception, the matter contained in the exception must be set up as a defense by the accused.
The Supreme Court undertook an elaborate study of the subject of pleading exemptions, and defendant called particular attention to the following excerpt: "where the exception, though in a subsequent clause or section, or even in a subsequent statute, is nevertheless clothed in such language, and is so incorporated as an amendment with the words antecedently employed to define the offence, that it would be impossible to frame the actual statutory charge in the form of an indictment with accuracy, and the required certainty, without an allegation showing that the accused was not within the exception contained in the subsequent clause, section, or statute. Obviously such an exception must be pleaded, as otherwise the indictment would not present the actual statutory accusation, and would also be defective for the want of clearness and certainty".
The Supreme Court said:
"Commentators and judges have sometimes been led into error by supposing that the words `enacting clause,' as frequently employed, mean the section of the statute defining the offense, as contradistinguished from a subsequent section in the same statute, which is a misapprehension of the term, as the only real question in the case is, whether the exception is so incorporated with the substance of the clause defining the offense as to constitute a material part of the description of the acts, omission, or other ingredients which constitute the offense. Such an offense must be accurately and clearly described, and if the exception is so incorporated with the clause describing the offense that it becomes in fact a part of the description, then it cannot be omitted in the pleading; but if it is not so incorporated with the clause defining the offense as to become a material part of the definition of the offense, then it is matter of defense and must be shown by the other party, though it be in the same section or even in the succeeding sentence.
"Few better guides upon the general subject can be found than the one given at a very early period, by Treby, Ch.J., in Jones v. Axen, 1 Ld. Raym. 120."