var preview_page = "{?$preview_page?}";
var next_page = "{?$next_page?}";
var index_page = "{?$index_page?}";
var article_id = "{?$article_id?}";
var chapter_id = "{?$chapter_id?}";
function jumpPage() {
  var event = document.all ? window.event : arguments[0];
  if (event.keyCode == 37) document.location = preview_page;
  if (event.keyCode == 39) document.location = next_page;
  if (event.keyCode == 13) document.location = index_page;
}
document.onkeydown=jumpPage;