docs: def make_exec(fname, version):
      """
      Writes the shebang and makes the file executable.
      """
      # if no version is specified, use system default.
      if version is None:
          version = 'default'
  
      # write the shebang and then make the file executable.
      with open(fname, 'rb+') as f:
          put_shebang(f, version)
      # make the file
      os.chmod(fname, os.stat(fname).st_mode | 0o0111)
      print("{} is now executable".format(fname))