anchor.diff:  -H "Content-Type: application/json" \
   -d '{
     "event" : "$set",
  -  "entityType" : "itemConstraint"
  -  "entityId" : "unavailable",
  +  "entityType" : "constraint"
  +  "entityId" : "unavailableItems",
     "properties" : {
       "items": [],
     }
  
positive.code:   <USER ID>, {
      'targetEntityType' => 'item',
      'targetEntityId' => <ITEM ID>
    }
  )
  
  
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">
  