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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 create_xref(self):
 """ Create Class, Method, String and Field crossreferences for all classes in the Analysis. If you are using multiple DEX files, this function must be called when all DEX files are added. If you call the function after every DEX file, the crossreferences might be wrong! """

        log.debug("Creating Crossreferences (XREF)")
        tic = time.time()

        # TODO on concurrent runs, we probably need to clean up first,
        # or check that we do not write garbage.

        # TODO multiprocessing
        for c in self._get_all_classes():
            self._create_xref(c)

        log.info("End of creating cross references (XREF)")
        log.info("run time: {:0d}min {:02d}s".format(*divmod(int(time.time() - tic), 60)))