var n = 11;
var pr_flag = false;
var b = new Array;
function prel(){
   	if(document.images) {
	  for(i=1;i<=n;i++)
      {
        str = 'buttons/b' + i + '_o.gif';
        b[i-1] = new Image();
        b[i-1].src = str;
      }

        i=20;
        str = 'buttons/b' + i + '_o.gif';
        b[i-1] = new Image();
        b[i-1].src = str;

    pr_flag = true;
    }
}
function on(im){
	if (document.images && (pr_flag == true)){
        document.images[im].src="buttons/"+im+"_o.gif";
    }
}

function off(im){
	if (document.images && (pr_flag == true)){
        document.images[im].src = "buttons/"+im+".gif";
    }
}