anchor.diff:     'targetEntityId' => <ITEM ID>
   ));
   
  +// Set a list of unavailable items
  +$client->createEvent(array(
  +  'event' => '$set',
  +  'entityType' => 'constraint',
  +  'entityId' => 'unavailableItems',
  +  'properties' => array('items' => array('<ITEM ID1>', '<ITEM ID2>'))
  +));
  +
  +
   ?>
   ```
     </div>
  
positive.code:   "entityId" : <USER ID>,
    "targetEntityType" : "item",
    "targetEntityId" : <ITEM ID>,
    "eventTime" : <TIME OF THIS EVENT>
  }'
  
  
negative.code: the E-Commerce Recommendation Engine Template supports 2 types of entities: **user** and
  **item**, events **view** and **buy**. An item has the **categories** property, which is a list of category names (String). A user can view and buy an item.
  
  There is also one specical **constraint** entiy with entityId **unavailableItems**, which defines a list of unavailable items and is taken into account in real time during serving.
  
  <%= partial 'shared/quickstart/install_sdk' %>
  
  The following is sample code of using different SDKs to import events of setting user and item's properties, and user-view-item events, user-buy-item event and setting the constraint unavailableItems.
  
  <div class="tabs">
    <div data-tab="Python SDK" data-lang="python">
  