duplica pari in lista unidirezionale

function duplica(l)
{
    var prev,tmp={};
    if(l==null){return;}
    for(prev=l.testa;prev!=null;prev=prev.succ)
    {
        if(prev.x%2===0)
        {       tmp={x:prev.x,succ:null};
                tmp.succ=prev.succ;
                prev.succ=tmp;
                prev=prev.succ;
        }
    }
}