var prev_selected;
var prev_obj;

function sel_panel(panel_id)
{
if(prev_selected == panel_id)
    {
    document.getElementById(panel_id).background = 'images/4.gif';
    document.getElementById("panel_"+panel_id).style.display = 'none';
    prev_selected = undefined;
    }
else
    {
    document.getElementById(panel_id).background = 'images/1.gif';
    if(prev_selected != undefined)
        {
        document.getElementById(prev_selected).background = 'images/4.gif';
        document.getElementById("panel_"+prev_selected).style.display = 'none';
        }
    document.getElementById("panel_"+panel_id).style.display = 'inline';
    prev_selected = panel_id;
    }
}