Stephanie was playing softball on a sunny Sunday afternoon when she was struck on the back of the head with a bat. After she regained consciousness, she learned that Donny had been the person who struck her. However, Donny argues that he would not have struck Stephanie had Tom not tripped him, causing him to swing the bat carelessly and strike her. Stephanie brings a lawsuit against Donny. Donny, arguing that he was not totally responsible for her injuries, consults her attorney for the best way to bring Tom into the lawsuit?