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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 files_exist(self):
 """Check if SlackBuild archive.tar.gz and sources exist """

        if not os.path.isfile(self.path + self.script):
            print("\nslpkg: Error: SlackBuild archive.tar.gz not found\n")
            raise SystemExit()
        for src in self.sources:
            if not os.path.isfile(self.path + src):
                print("\nslpkg: Error: Source file '{0}' not found\n".format(
                    src))
                raise SystemExit()