> The draft, as written, doesn't seem to allow  #x789_AB_CD_EF .
>
> Specifically:
>> Underscores between letters a-z A-Z that represent digits in a radix
>> higher than 10 (using the standard hexadecimal
>> read syntax, or any implementation-defined read syntax).
>
> does not handle underscores between letters and digits.
>
> This needs some word-smithing.
>
> It wouldn't hurt to add an example, as well.

It does allow that since letters in a number with radix higher than 10
are considered digits for the purpose of the rule. The "includes" and
"excludes" lists are meant to illustrate the implications of the rule
and are not necessarily exhaustive (due to lack of ability on my part -
I agree that it would be best if they were exhaustive).

But you are right - the illustration could be clearer. I'll change it as
you suggest and add an example as well. Thanks for reading closely and
commenting!