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Very simple. Wikipedia is licensed under CC-BY-SA 3.0. If you add Public Domain entries without identifying them as such you are in effect removing them from PD and making them CC-BY-SA. That's a contradiction - public domain ""stuff"" cannot be pulled back to a more restrictive licence (which CC-BY-SA, GNU et.al are). If it's PD it's quoted/intact or re-written / modified.    Chat   "