In PoserPython, is there any universal 'stop this script here, progress no further' command?

For instance:

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   actor = scene.CurrentActor()
   # If no light is currently selected in the scene, then display this message and stop the script.
   if not actor.IsLight():
      poser.DialogSimple.MessageBox("To run this script, please first select a light.")
      >>> STOP HERE! <<
   else:
      if actor.IsLight():
   # OK, we have a light selected, so continue running the script.
          lights = scene.Lights()
          for light in lights:

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What needs to go on the line >>> STOP HERE! << to cause the script to exit gracefully? I see "Return" used in other scripts, but that fails to prevent the script from continuing to run.

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