code: public boolean restricted(Order<?, ?> order) {
  		if (!order.property().metamodel().isRoot()) {
  			restricted = true;
  			return true;
  		}
  		
  		if (restricted) {
  			return true;
  		}
  		
  		if (this.order == null) {
  			if (filter != null && filter.property() != order.property()) {
  				restricted = true;
  				return true;
  			}
  			
  			this.order = order;
  		}
  		
  		return false;
  	}
docs: Indicates whether the specified {@code Order} is restricted to Google App
  Engine datastore query or not.
  This method indicates the following cases as <b>restricted</b>:
  <ul>
  <li>
  Property that the specified order targets is not a root entity's property.
  </li>
  <li>
  If any inequality filter has been already specified, the property that
  the filter targets is not same as the one that the first-qualified order
  targets.
  </li>
  <li>
  All orders after the order indicated as restricted once.
  </li>
  </ul>
  
  @param order The order to be indicated whether restricted or not.
  @return Whether the specified {@code Order} is restricted to Google App
  Engine datastore query or not.