function abrirventana(pagina, nombreventana, width, height, windowfeatures) {
  var winl = (screen.width - width) / 2;
  var wint = (screen.height - height) / 2;
  windowfeatures += ', width='+width+',height='+height+',top='+wint+',left='+winl;
  window.open(pagina, nombreventana, windowfeatures);
}