title: Where is the setCouponCode function is defined in Magento
body: <p>I know that setCouponCode is catch inside __call in Varien_Object but I can't figure out where it is defined.</p>
  
  <p>I need it because I want to show the coupon code even if the discount equal zero.
  I believed it's done inside this function.</p>
  
  <p>So if anybody know where the function is defined or where I can modify the code to get the coupon code displayed all the time, please let me know.</p>