That's only meaningful for base-2.

Well, that's how computers work, don't they... since the whole business of this convoluted module is only yielding the string representation of a value $n to base $b via a charset @c, while all calculations are carried out on the number/integer slots of its objects - it doesn't really calculate in base $b (no such machine except $base == 2) - the only thing is to find a good stringified representation of the underlying numbers, whilst not limiting the native number range.

So I guess the best way to go here is outlawing '-' from the valid chars array and using that as a prefix for negative numbers. That would reduce the maxbase to 90, which I deem not to be a big loss. Would that be a good way to go?