The 639-3 code set incorporates the
639-2/T code elements, and only lists the 639-2/B for completeness of reference.
I don't believe there is any incompatibility with ISO 639-3 caused by this
proposal. The macrolanguage Serbo-Croatian is [hbs] and has [hrv] Croatian,
[srp] Serbian, and [bos] Bosnian as its member languages (the terminological