I think you'd have to ask Don about that.  AFAICT the basic situation is that the function probably needs to be generic with respect to the type of `BigDigit` (effectively the "word type" for the integer).  However, it's a bit of a moot point right now as all the implementations assume that the word type is `uint`.

Theoretically at least I'd imagine that on 64-bit systems it would be better to use `ulong`.