document.onkeydown = checkKeycode;
/* navigate to prev/next page using keyboard <- and -> */
function checkKeycode(e) {
	var keycode;
	if (window.event) keycode = window.event.keyCode;
	else if (e) keycode = e.which;
	//alert("keycode: " + keycode);
	switch(keycode){
		case 37: // <-
			try{
				var previous_url = document.getElementById('previous_page_btn').href;
				//alert(previous_url);
				window.location.href = previous_url;
			}catch(e){}
			break;
		case 39: // ->
			try{
				var next_url = document.getElementById('next_page_btn').href;
				//alert(next_url);
				window.location.href = next_url;
			}catch(e){}
			break;
	}
}