DALLAS, TX - APRIL 25:  In this handout provided by the George W. Bush Presidential Center, former U.S. presidents (L-R) Jimmy Carter, Bill Clinton, George H.W. Bush, George W. Bush and President Barack Obama pose at the opening of the George W. Bush Presidential Center April 25, 2013 in Dallas, Texas.  (Photo by Paul Morse/George W. Bush Presidential Center via Getty Images)