function AplicarCebra () {
   var tables = document.getElementsByTagName("table");
   for (var i = 0; i < tables.length;   i++) {
   if (tables[i].className.match(/TablaCebra/))   {
   TablaCebra(tables[i]);
   }
 }
 }
 function TablaCebra (table) {
   var current =   "impar";
   var trs = table.getElementsByTagName("tr");
   for (var i = 0; i   < trs.length; i++) {
   trs[i].className += " " + current;
   current =   current == "par" ? "impar" : "par";
   }
  }