Get a set of objects specifyng its ids on a JSON GET call

I'm developing a JSON WCF Web Service with C#.

I've just defined these two urls:

http://localhost:39325/RestServiceImpl.svc/forms
 
 http://localhost:39325/RestServiceImpl.svc/forms/{id}
 

To retrieve to get all forms or a form with an {id}.

But now, I need to another one: I want to get all forms whose IDs are not included on a set. In other words, on client I will have some forms. I need to get new forms only (those which are not yet on client).

How can I specify an ids list on JSON url?

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