function toggleVisible(elem) {
   toggleElementClass("invisible", elem);
}

function makeVisible(elem) {
   removeElementClass(elem, "invisible");
}

function makeInvisible(elem) {
   addElementClass(elem, "invisible");
}

function isVisible(elem) {
   // you may also want to check for
   // getElement(elem).style.display == "none"
   return !hasElementClass(elem, "invisible");
};