As the nation tries to come to grips with the senseless bloodshed in Boston, there are heartening reports from New York about a remarkable outbreak of empathy among arch rivals. It has nothing to do with Republicans and Democrats. Rather, on Tuesday, a stadium full of New York Yankees fans stood arm-in-arm to sing along to “Sweet Caroline,” the anthem of their bitter baseball rivals, the Boston Red Sox. “Everybody in New York knows what they’re going through,” said one hard-bitten Yankees fan. He recalled that after the 9/11 attacks on his city, fans in Boston were singing “New York, New York.” Definitive evidence that we’re all Americans first.