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Q. Hello, Chicago. You state that “an opening parenthesis should be preceded by a comma or a semicolon only in an enumeration” as in (1) a brown fox, (2) a silver fox. There are no other exceptions. You also say that the same rules apply to brackets. Another editor wants this: New Westminster, BC: Pie Tree Press, . It looks very wrong to me! I say the comma goes, because the bracketed matter is an interpolation, not part of the original text, and the comma has no function. Therefore the punctuation should be as if that interpolation doesn’t exist.
A. Although you could omit the comma or put it inside the brackets, it reads most smoothly the way you’ve shown it. A citation isn’t a quotation. The comma is being supplied by the author who’s writing the citation. In fact, citations in CMOS show a comma before a bracketed date in just this way, so yes, this is another exception to the general rule.