anchor.diff:    <USER ID>, {
       'targetEntityType' => 'item',
       'targetEntityId' => <ITEM ID>
  +  }
  +)
  +
  +# Set a list of unavailable items
  +client.create_event(
  +  '$set',
  +  'constraint',
  +  'unavailableItems', {
  +    'properties' => { 'items' => ['<ITEM ID1>', '<ITEM ID2>'] }
     }
   )
   
  
positive.code: -H "Content-Type: application/json" \
  -d '{
    "event" : "$set",
    "entityType" : "itemConstraint"
    "entityId" : "unavailable",
    "properties" : {
      "items": [],
    }
  
negative.code: 
  The template has the following features:
  
  - handle out of stock items
  - recommend items to new user who signs up after the model is trained.
  - recommend unseen items only.
  
  