var active = new Array();
active['o'] = 1;
active['m'] = 1;
active['u'] = 1;

var max_number = new Array();
max_number['o'] = 7;
max_number['m'] = 7;
max_number['u'] = 7;

function swap(what)
{
   var i;
       
   for(i = 0; i < max_number[what]; i++){
       var e = document.getElementById(what + i);
       e.style.visibility = (i == active[what]) ? "visible" : "hidden";
   }
   active[what]++;    
   active[what] %= max_number[what];    
}