code: def _reachable_subsystems(network, indices, state):
      
      validate.is_network(network)
  
      # Return subsystems largest to smallest to optimize parallel
      # resource usage.
      for subset in utils.powerset(indices, nonempty=True, reverse=True):
          try:
              yield Subsystem(network, state, subset)
          except exceptions.StateUnreachableError:
              pass
docs: A generator over all subsystems in a valid state.