title: How can I get a specific element of a set in GAMS
body: <p>Suppose we have the following code:</p>
  
  <pre><code>sets
  i index of products /product_1, product_2/;
  
  parameters
  Inventory_Cost(i) /product_1 3, product_2 4/;
  
  </code></pre>
  
  <p>Now when I want to use <code>Inventory_Cost(3-i)</code> it gives me an error. <code>Inventory_Cost(3-ord(i))</code> doesn't work also. How should i correct this?</p>