Dan Mulhall’s final diplomatic posting in a distinguished and prodigious career taught him many things – about politics, trade, and, of course, Trump. But it also taught him about retirement.   One lesson came from Alan Greenspan, the American economist who served as the 13th chairman of the Federal Reserve from 1987 to 2006. Sitting beside Greenspan, then well into his nineties, at a dinner, Mulhall asked Greenspan how he was occupying his time. “I’m writing an economic history of America,” Greenspan responded deadpan. Another came from George Shultz, a former US secretary of state who delivered a speech on Brexit…