The persistent id you get for a track from iTunes via AppleScript is a floating point number (something like 3.42741835566726E+18), but in the XML file, for example, it is represented as a hexadecimal string (something like 2F90A2D5D2334B1A). Same for playlists. I didn't spend a lot of time on this when persistent id was introduced, but at the time I recall I wasn't able to find a conversion routine for floating point numbers--or anyway a way of conveniently using them. You are right not to depend on database ids as these can change. So, if you can manage to find a way of working with those kinds of numbers it may be beneficial.
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