Returns data with different cfgstr values that were previously computed
        with this cacher.

def existing_versions(self):
        """
        Returns data with different cfgstr values that were previously computed
        with this cacher.
        """
        import glob
        pattern = self.fname + '_*' + self.ext
        for fname in glob.glob1(self.dpath, pattern):
            fpath = join(self.dpath, fname)
            yield fpath