I have a question about the std::error type.
Can anybody tell me whether it is supposed to be extensible or not?

   - If yes, how do I use and extend it with my own "domains", in
   particular, how it will be guaranteed that there will not be any collisions
   in the values of "domain"?
   - If no (ie std::error will only be able to represent well-defined
   classes of errors curated by std committee), then I do not understand how
   it can be useful for the public.