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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def backtrack(self, decision_level):
 # type: (int) -> None """ Resets the current decision level to decision_level, and removes all assignments made after that level. """

        self._backtracking = True

        packages = set()
        while self._assignments[-1].decision_level > decision_level:
            removed = self._assignments.pop(-1)
            packages.add(removed.dependency.name)
            if removed.is_decision():
                del self._decisions[removed.dependency.name]

        # Re-compute _positive and _negative for the packages that were removed.
        for package in packages:
            if package in self._positive:
                del self._positive[package]

            if package in self._negative:
                del self._negative[package]

        for assignment in self._assignments:
            if assignment.dependency.name in packages:
                self._register(assignment)