unwind(halt(status)). 
If this is not possible, ignore this flag. Return false.
Unless PL_HALT_WITH_EXCEPTION was specified and 
effective, Clean up the Prolog environment using PL_cleanup() 
and if successful call exit() with the status argument. Returns true 
if exit was cancelled by PL_cleanup().239Versions 
up to 9.3.12 returned false on cancel. If necessary, the 
two behaviours can be distinguished based on the existence of the PL_HALT_WITH_EXCEPTION 
macro.