function add_comp_prod(valor){

		var prods = document.getElementById("selec").value;
		var item = document.getElementById(valor);
		var product="";
		
		if(item.checked == true){
		    if (prods == "")
		    	prods = valor;
		    else
		    	prods += ","+valor;
		    document.getElementById("selec").value = prods;
		}else{
			p = prods.split(",");
			for (i=0; i<p.length;i++){
				if (p[i]!= valor){
					if(product == "")
						product = p[i];
					else
						product += ","+p[i];
				}
			}
			document.getElementById("selec").value = product;
		}
		
		
}

function comparar_prod(form){
	if (!document.getElementById('selec') ||  document.getElementById("selec").value == "" ){
		alert("Debe seleccionar productos a comparar.");
		return;
	}
	form.prod_marcado.value = document.getElementById("selec").value;
	
	form.submit();
}