Song Tao, a prominent diplomat and head of the International Liaison Department of the Communist Party of China (CPC), arrived in North Korea on Friday for a four-day visit as a special envoy of Beijing.Song Tao’s trip comes at a time when relations between North Korea and China are changing fast: since mid-August, Beijing has been increasingly tough on its neighbor and, technically, ally.Readers with good memories may recall that we have seen this before: in recent years there have been many episodes when observers claimed that China had finally decided to get serious about economic sanctions