George W. Bush Presidential Center

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In this image from video posted on Facebook, courtesy of the George W. Bush Presidential Center, former President George W. Bush participates in the ice bucket challenge with the help of his wife, Laura Bush, in Kennebunkport, Maine. The challenge has caught on with notable figures participating in the campaign to raise money for the fight against ALS, or Lou Gehrig's disease. (AP Photo/Courtesy George W. Bush Presidential Center)

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Former President George W. Bush makes remarks during the Monetary Policy and the Economy, a conference examining central banking impact on prosperity on Monday, May 19, 2014, at the George W. Bush Presidential Center in Dallas. (AP Photo/The Dallas Morning News, David Woo) MANDATORY CREDIT; MAGS OUT; TV OUT; INTERNET USE BY AP MEMBERS ONLY; NO SALES.

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Margaret Spellings, left, Charity N. Wallace, second from left, Hadeer Maher, and Pam Allyn, right, speak in a moderated conversation during George W. Bush Institute's fellowship program on International Women's Day at the George W. Bush Presidential Center, March 7, 2014, in Dallas. (AP Photo/The Dallas Morning News, Sarah Hoffman) MANDATORY CREDIT; MAGS OUT; TV OUT; INTERNET USE BY AP MEMBERS ONLY; NO SALES

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Former Vice President Dick Cheney attends the dedication of the George W. Bush Presidential Center in Dallas on Thursday, April 25, 2013. (AP Photo/David J. Phillip)