code: def _add_proposed():
      
      release = get_distrib_codename()
      arch = platform.machine()
      if arch not in six.iterkeys(ARCH_TO_PROPOSED_POCKET):
          raise SourceConfigError("Arch {} not supported for (distro-)proposed"
                                  .format(arch))
      with open('/etc/apt/sources.list.d/proposed.list', 'w') as apt:
          apt.write(ARCH_TO_PROPOSED_POCKET[arch].format(release))
docs: Add the PROPOSED_POCKET as /etc/apt/source.list.d/proposed.list
  
      Uses get_distrib_codename to determine the correct stanza for
      the deb line.
  
      For intel architecutres PROPOSED_POCKET is used for the release, but for
      other architectures PROPOSED_PORTS_POCKET is used for the release.