TEXT: Mohamedou Ould Slahi, a Guantanamo Bay prisoner who wrote a best-selling book about his experiences in the military prison, poses on October 18, 2016 in Nouakchott, after he was reunited with his family in his native Mauritania on October 17 after 14 years of detention   The transfer of Mohamedou Ould Slahi, believed to be the last inmate from Mauritania held at the facility in Cuba, brings the prison's remaining population down to 60. His case became a cause celebre after the publication last year of