returns all line numbers that are used in a jump.

def get_destinations(self, ascii_listing):
        """
        returns all line numbers that are used in a jump.
        """
        self.destinations = set()
        def collect_destinations(matchobj):
            numbers = matchobj.group("no")
            if numbers:
                self.destinations.update(set(
                    [n.strip() for n in numbers.split(",")]
                ))

        for line in self._iter_lines(ascii_listing):
            self.renum_regex.sub(collect_destinations, line)

        return sorted([int(no) for no in self.destinations if no])