function change_img(src, id) {
    document.getElementById(id).src = src;
	return;
}

var lastVisibleElemId;
function showElem(id) {
    document.getElementById(id).style.visibility = "visible";
    if (lastVisibleElemId != null) {
        hideElem(lastVisibleElemId);
    }
    lastVisibleElemId = id;	return;
}

function hideElem(id) {
    document.getElementById(id).style.visibility = "hidden";
    lastVisibleElemId = null;	return;
}

function setElemPosition(id) {
//     getOffset();
    document.getElementById(id).style.top = (window.pageYOffset + 50) + "px";
	return;
}

function getOffset() {
    alert("Y offset: " + window.pageYOffset);	return;
}