function show_ctrl() {
	if (document.getElementById("ctrl_up")) document.getElementById("ctrl_up").style.visibility = "visible";
	if (document.getElementById("ctrl_next")) document.getElementById("ctrl_next").style.visibility = "visible";
	if (document.getElementById("ctrl_prev")) document.getElementById("ctrl_prev").style.visibility = "visible";
}
function hide_ctrl() {
	if (document.getElementById("ctrl_up")) document.getElementById("ctrl_up").style.visibility = "hidden";
	if (document.getElementById("ctrl_next")) document.getElementById("ctrl_next").style.visibility = "hidden";
	if (document.getElementById("ctrl_prev")) document.getElementById("ctrl_prev").style.visibility = "hidden";
}