code: def get_third_party(self, third_party):
          
          actual_account = third_party.get_account()
  
          assert actual_account.get_bookset() == self.get_bookset()
          return ProjectAccount(actual_account, project=self, third_party=third_party)
docs: Return the account for the given third-party.  Raise <something> if the third party doesn't belong to this bookset.