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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 subsystems(self):
 """Returns all subsystem types used by tasks in this goal, in no particular order."""

    ret = set()
    for task_type in self.task_types():
      ret.update([dep.subsystem_cls for dep in task_type.subsystem_dependencies_iter()])
    return ret