code: @Override
  	public <E, R> AppEngineGetScalar<E, R> get(Aggregation<R> aggregation) {
  		if (aggregation == null) {
  			throw new IllegalArgumentException("'aggregation' must not be [" + aggregation + "]");
  		}
  		
  		return new AppEngineGetScalar<E, R>(aggregation, this);
  	}
docs: Returns {@code GetScalar} command based on App Engine Low-level Datastore
  API for the specified {@code Aggregation}.
  This method is the entry point for aggregation Command Builder API.
  
  @param <E> The entity type.
  @param <R> The property value type to aggregate with this
  {@code GetScalar}.
  @param aggregation The {@code Aggregation} this method executes.
  @return {@code GetScalar} for the specified {@code Aggregation} instance.