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the QUIT hook is actually not that trivial and required
some architectural changes.
First, the QUIT hook execution and any error that might
occurr cannot always be attached to an existing error stack
frame. Thereforce, to give a stack frame for QUIT hooks and
to improve the readability of error traces for ED hooks in general,
a special EDHookFrame is added to every ED hook execution error.
Secondly, since QUIT hooks can themselves throw errors, we cannot
run it from an atexit() handler. Instead it's always called manually
before __successful__ program termination. An error in a QUIT hook
will result in a failure return code nevertheless.
Thirdly, errors in QUIT hooks should not prevent program termination
(in interactive mode), therefore they are only invoked from main()
and always in batch mode. I.e. if the interactive mode is terminated
(EX$$), SciTECO will switch back to batch mode and run the QUIT
hook there. This is also symmetric to program startup, which is
always in batch mode.
This means that Interface::event_loop() no longer runs indefinitely.
If it returns, this signals that the interface shut down and
batch mode may be restored by SciTECO.
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