docs: def diffusion_driver(self):
          """ diffusion driver are the underlying `dW` of each process `X` in a SDE like `dX = m dt + s dW`
  
          :return list(StochasticProcess):
  
          """
          if self._diffusion_driver is None:
              return self,
          if isinstance(self._diffusion_driver, list):
              return tuple(self._diffusion_driver)
          if isinstance(self._diffusion_driver, tuple):
              return self._diffusion_driver
          return self._diffusion_driver,