var p = 0; /* p = previously opened element. */
function show_mnu(id){
var e = document.getElementById(id);
e.style.display = 'block';
hide_mnu(id);
p = id;
}
function hide_mnu(id){
if(p != 0){
var e = document.getElementById(p);
e.style.display = 'none';
}
}