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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def add_backend(self, backend):
        "Add a RapidSMS backend to this tenant"
        if backend in self.get_backends():
            return
        backend_link, created = BackendLink.all_tenants.get_or_create(backend=backend)
        self.backendlink_set.add(backend_link)