>     But unless the programmer is catching that error, it may lead to an
>     exit with a different exit code
>
> <http://www.tldp.org/LDP/abs/html/exitcodes.html> says that exit code 128 should be used for this purpose.  Additional exit code conventions can be found at /usr/include/sysexit.h, but AFAIK nobody much uses them

Yeah, <sysexits.h> is a dead letter and the range of situations it
covers is too narrow/vague to be useful anyway (EX_SOFTWARE is the
EINVAL of exit codes: a category covering everything and nothing).

Is 128 fully portable? For the longest time, I thought the safe portable
maximum was 126 (with 127 reserved for some kind of abnormal
termination) but that information may be more than a decade old. Current
safe range would seem to be 0..255, I can't find any concrete exceptions
to that rule.

> Of course, R7RS (and R6RS too) are deliberately silent on what happens when an exception is not caught (return to the REPL, return to the shell, assert a pin and halt the CPU, etc.)

Nasal demons?