President Obama stands with former presidents George W. Bush, Bill Clinton, George H.W. Bush, and Jimmy Carter at the dedication of the George W. Bush presidential library on the campus of Southern Methodist University in Dallas, Thursday, April 25, 2013. (AP Photo/Charles Dharapak)

Photo by: Charles Dharapak

President Obama stands with former presidents George W. Bush, Bill Clinton, George H.W. Bush, and Jimmy Carter at the dedication of the George W. Bush presidential library on the campus of Southern Methodist University in Dallas, Thursday, April 25, 2013. (AP Photo/Charles Dharapak)