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The concept of a GroupName might also be useful for other nice-to-have but not critical features like grouping several categories together in a single file. In particular I am thinking of categories on NSValue to work with new struct types. For example valueWithPoint: is defined in NSValue (NSValueGeometryExtensions) in NSGeometry.h and valueWithRange: is defined in NSValue (NSValueRangeExtensions) in NSRange.h yet they are documented in the same file. Similarly, I have a categories on NSValue for my own types that are defined is several different headers, and while it would not be the end of the world if each category appeared in it's own file, being able to define a ClassName GroupName might make it possible to have them all in the same file.
(suggested by spalte - see #2)
I think it would be better to enable this through command line switch similar to --merge-categories or even reuse the same switch. This would give less control, but would be fully automated and simpler to implement. Also there's ussually preference to do it either one way or the other.
Browsing through older issues, realized this one is already implemented - as described here