$(document).ready(function(){
	$(".principal li").hover(
      function () {
		$(this).find('ul').css("display","block");
		$(this).find('ul li a img').css("opacity",0);
      }, 
      function () {
		$(this).find('ul').css("display","none");
		$(this).find('ul li a img').css("opacity",0);
      }
    );
});

function isset(variable_name) {
    try {
         if (typeof(eval(variable_name)) != 'undefined')
         if (eval(variable_name) != null)
         return true;
     } catch(e) { }
    return false;
   }