function hideLayer(whichLayer) {

if (document.getElementById) {
// this is the way the standards work
document.getElementById(whichLayer).style.visibility = "hidden";
}
else if (document.all) {
// this is the way old msie versions work
document.all[whichlayer].style.visibility = "hidden";
}
else if (document.layers) {
// this is the way nn4 works
document.layers[whichLayer].visibility = "hidden";
}

}

function showLayer(whichLayer) {

if (document.getElementById) {
// this is the way the standards work
document.getElementById(whichLayer).style.visibility = "visible";
}
else if (document.all) {
// this is the way old msie versions work
document.all[whichlayer].style.visibility = "visible";
}
else if (document.layers) {
// this is the way nn4 works
document.layers[whichLayer].visibility = "visible";
}

}

function handleClick(whichClick) {

if (whichClick == "close_all") {
// then the user wants to hide the layer
hideLayer("bezit");
hideLayer("recreatie");
hideLayer("gezondheid");
hideLayer("mobiliteit");

}
else if (whichClick == "bezit") {
// then the user wants to show the layer
showLayer("bezit");
hideLayer("recreatie");
hideLayer("gezondheid");
hideLayer("mobiliteit");
}

else if (whichClick == "recreatie") {
// then the user wants to show the layer
hideLayer("bezit");
showLayer("recreatie");
hideLayer("gezondheid");
hideLayer("mobiliteit");
}
else if (whichClick == "gezondheid") {
// then the user wants to show the layer
hideLayer("bezit");
hideLayer("recreatie");
showLayer("gezondheid");
hideLayer("mobiliteit");
}
else if (whichClick == "mobiliteit") {
// then the user wants to show the layer
hideLayer("bezit");
hideLayer("recreatie");
hideLayer("gezondheid");
showLayer("mobiliteit");
}

}