The killing of three U.S. doctors Thursday, allegedly by an Afghan policeman guarding their hospital, raises anew questions about the wisdom of a continued U.S. presence there, in uniform, scrubs or any other kind of garb. While U.S. troops may have increased protection after a spate of so-called blue-on-green attacks in recent years, the lifesavers working at Kabul’s Cure International Hospital apparently were slain by a policeman dedicated to their protection. The murders come as two veteran reporters file on what life is like in Iraq, where the last U.S.