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geonames.usgs.gov/pls/gnispublic/f?p=gnispq:3:::NO::P3_FID:829697
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www.gretnanebraska.com/ (9 Aug 2010)
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www.usgennet.org/usa/ne/county/sarpy/gretna.htm (15 Dec 2009)
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en.wikipedia.org/wiki/Gretna,_Nebraska (10 Aug 2010)
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geonames.usgs.gov/pls/gnispublic/f?p=gnispq:3:::NO::P3_FID:2394248