function toggleLayer( whichLayer ){
	var elem, vis;
	if( document.getElementById ) // this is the way the standards work
		elem = document.getElementById( whichLayer );
	else if( document.all ) // this is the way old msie versions work
		elem = document.all[whichLayer];  
	else if( document.layers ) // this is the way nn4 works
		elem = document.layers[whichLayer];  
	vis = elem.style;
	// if the style.display value is blank we try to figure it out here
	if(vis.display==''&&elem.offsetWidth!=undefined&&elem.offsetHeight!=undefined)
		vis.display = (elem.offsetWidth!=0&&elem.offsetHeight!=0)?'block':'none';
	vis.display = (vis.display==''||vis.display=='block')?'none':'block';
}

function elemOn(elem_id){
	if(document.getElementById(elem_id))
		document.getElementById(elem_id).style.display = "block";
}

function elemOff(elem_id){
	if(document.getElementById(elem_id))
		document.getElementById(elem_id).style.display = "none";
}

function hideAll(){
	for( var i = 1; i <= 15; i++ )
		elemOff( 'tanc'+i );
}

function showAll(){
	for( var i = 1; i <= 15; i++ )
		elemOn( 'tanc'+i );
}