function remove_duplicates_filter () {
    var acc = {};
    return function (x) {
        if (acc[x]) return false;
        acc[x] = 1;
        return true;
    };
}

function magick_options_completer (input, cursor_position, conservative) {
  var completions;
  try {
    completions = get_pref ('conkeror.webjump.magick-options.cache').split (' ');
  } catch (e) { }
  if (! completions) {
    try {
      var content = yield send_http_request (
        load_spec({uri: "http://imagemagick.org/script/command-line-options.php"}));
      completions = content.responseText.match(/([a-z]+)(?=\">-\1<\/a>)/g).filter (remove_duplicates_filter ());
      user_pref ('conkeror.webjump.magick-options.cache', completions.join (' '));
    } catch (e) { }
  }
  yield co_return (
    prefix_completer ($completions = completions)
    (input, cursor_position, conservative));
}

define_webjump (
    "magick-options",
    "http://imagemagick.org/script/command-line-options.php#%s",
    $completer = magick_options_completer);

clear_pref ('conkeror.webjump.magick-options.cache');