Semantics of toString

[Don]

bearophile wrote:
> Bill Baxter:
> 
>> Though they may be useful, those don't look to have anything to do
>> with formatting user types into strings, which is the subject at hand.
> 
> Don has said: "But the performance would still be very poor, and that's much more difficult to solve." And those links show a way to quickly convert a large multi-precision integer into a string. What is that I am missing?

It's problem 2 from my original posts: being able to output something 
large (eg an xml doc) in a piece-by-piece manner.