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