/*-- obre finestra posicionada al centre de la pantalla */
function obre(desti,nom,am,al){
  var ampla = am;
  var altura = al;
  strURL = desti;
  poshoritz = (screen.width) ? (screen.width-ampla)/2 : 0;
  posvert = (screen.height) ? (screen.height-altura)/2 : 0;
  strW = 'toolbar=0,directories=0,location=0,status=no,menubar=0,resizable=1,copyhistory=0,scrollbars=0,width='+ ampla + ',height='+ altura + ',top='+ posvert + ',left='+ poshoritz;
  eval("window.open('" + strURL + "',nom,'" + strW + "')");
}