You're right it can't be done (Perl 6 does that a lot). The feature could be made to start a new mapping of synthetic characters before it runs out, and throw

You're right it can't be done (Perl 6 does that a lot). The feature could be made to start a new mapping of synthetic characters before it runs out, and throw away the mapping when the last of the old strands gets GCed. Basically it is a problem in theory, but might not be in practice.