title: Override Mage_Catalog_Model_Resource_Product_Collection
body: <p>I'm trying to override this class in order to change the behavior of the function </p>
  
  <p><code>_applyProductLimitations</code></p>
  
  <p>and add another new function (so i can filter by two or more categories)</p>
  
  <p>Anyone can help telling me how to override it in order that all the product collection can call my new function without getting the error that it's not defined in </p>
  
  <p><code>Mage_Catalog_Model_Resource_Product_Collection</code> ?</p>
  
  <p>Thanks :)</p>