OSSP var

 o problem:
   ${foo:s/^(.+)$/<\1>/} 
   -> fails with malformatted search & replace error because
      it tries to parse the $ as a variable.
   ${foo:s/^(.+)\$/<\1>/} 
   -> does not substitute because it leaves the \$ in the regex
      and this no longer matches the end (instead it matches a dollar)

 o "problem"!?:
   ${unknown:-foo} fails on force_expand and results
   in no expansion (kept as is) if not forced_expand.
   It only does the expected if the callback returns
   empty strings for undefined variables. This is certainly
   intended this way, but has to be documented better?

 o use internal context to carry information

 o document exception handling

 o additional feature: functions via callback.
   ${foo:F<bar>:F<quux>} calls callback for bar with result of foo
   expansion and then callback for quux with the result returned from
   bar.