function update_qnt(arg) { 
var oldvalue = eval(document.cart_quantity.product_quantity.value); 
var next = 0; 
if(oldvalue == 1 && arg == -1) { 
document.cart_quantity.product_quantity.value = 1; 
next = 1; 
} 
if(oldvalue == 99 && arg == 1) { 
document.cart_quantity.product_quantity.value = 99; 
next = 1; 
} 
if(next == 0) 
document.cart_quantity.product_quantity.value = oldvalue + arg; 
}