Should store the data to the app's filesystem (sandbox), returning a unique identifier The returned UID has to be managed by the caller in order to retrieve the persisted data or to overwrite the contents using a successive storeData: call. 
      Syntax
         
         
            - 
            (
            NSString *
            ) 
            
               storeData
            
            : 
            (
            NSData *
            ) 
            data_in 
            
               withId
            
            : 
            (
            NSString *
            ) 
            uid_in
         
      
      Parameters
         
         
            - 
               data_in – 
               data to be persisted
            
 
            - 
               id_in – 
               should be nil when storing the data for the first time, or an existing ID to overwrite previously stored content
            
 
         
       
      Returns
         unique identifier (has to be managed by the caller in order to retrieve the persisted data)