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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 set_default_backend(self, backend_name):
 """Set the default backend of this circuit. This setting is only applied for this circuit. If you want to change the default backend of all gates, use `BlueqatGlobalSetting.set_default_backend()`. After set the default backend by this method, global setting is ignored even if `BlueqatGlobalSetting.set_default_backend()` is called. If you want to use global default setting, call this method with backend_name=None. Args: backend_name (str or None):
 new default backend name. If None is given, global setting is applied. Raises: ValueError: If `backend_name` is not registered backend. """

        if backend_name not in BACKENDS:
            raise ValueError(f"Unknown backend '{backend_name}'.")
        self._default_backend = backend_name