Having Nd be limited to characters that a) are used in decimal radix numbers b) are part of a complete, ordered sequence 0..9
would make this property regular enough to serve implementers. You could script the creation of relevant data for your implementation based on that property. *Exceptions* exist and need to be documented. Having exceptions machine readable is not as important, but having implementers understand them is. Therefore, the best thing is for these to become something other than Nd, but to retain their numeric type of digit. Together with a detailed explanation of each in the appropriate script chapter, AND a complete summary of all exceptional cases in a central place (section 4.6 comes to mind) would provide implementers with the information they need. The exceptional cases that I'm aware of are a) Arabic using two complete series of digits b) New Thai Lue using an extra digit 1 c) Han digits being scattered and used in two different types of numeric expressions d) ASCII digits being used for some scripts as preferred decimal-radix digits, because their native number system is not, or not exclusively decimal-radix The above information belongs in section 4.6 in summary form, or simply as table of pointers to each script chapter that contains a description of unusual numeric behavior for decimal-radix digits. (A separate table pulling together all the descriptions of non-decimal radix number systems that are discussed in the Standard would equally be useful for the readers). A./
