Long::Jump essentially provides a multi-level return. You can mark a spot with
setjump() and then unwind the stack back to that point from any nested stack
frame by name using longjump(). You can also provide a list of return values.
This is not quite a match for C's long jump, but it is "close enough". It is
safer than C's jump in that it only lets you escape frames by going up the
stack, you cannot jump in other ways.
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