// effect when mouse hovers over links in the navigation bar
function doHover() {
    event.srcElement.parentElement.style.backgroundColor = "gray";
    event.srcElement.parentElement.style.border = "thin inset gray";
}

// effect when mouse hovers outside links in the navigation bar
function undoHover() {
    event.srcElement.parentElement.style.backgroundColor = "black";
    event.srcElement.parentElement.style.border = "thin outset gray";
}

// reference event handlers for hover and hightlight effects
function initEffects() {
    for(var i = 0; i < 4; i++) {
        document.links[i].onmouseover = doHover;
        document.links[i].onmouseout = undoHover;
    }
}