docs: 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()