IF only North Korea could be there too, the gathering would have been complete. This week's overlapping visits to China by the leaders of Cuba and Vietnam mean that virtually every country of any consequence which still embraces communism will be represented in Beijing. China's Foreign Ministry claims this is by coincidence rather than design. Whatever the reason, not so long ago a gathering of all the communist leaders would have represented almost half the world. Now the collapse of the Soviet bloc has reduced their number to a handful. Those that survive are anxious to copy patriarch Deng Xiaoping's recipe for remaining in power. Cuban President Dr Fidel Castro's first visit to China, which begins on Wednesday, is testimony to how his economically crippled regime has at last realised its one hope of remaining in power lies in emulating Beijing's model of economic reform without political liberalisation. Vietnam learnt that lesson some years ago and instituted its own policy of limited reforms. But these lag far behind China's, whose economic success is increasingly causing headaches for Hanoi. Communist Party Chief Do Muoi, who arrived in Beijing yesterday at the start of a six-day visit, now has to contend with an economy flooded with cheap imports from the mainland, against which Vietnamese businesses are unable to compete. In different ways, Cuba and Vietnam are suffering from having no leader with the same vision as Mr Deng. His recognition, back in the late 1970s, that the only way a communist regime could expect to survive is by giving its citizens the same prosperity they could enjoy under capitalism allowed China to launch its reforms long before the collapse of the Soviet bloc. Other surviving communist states are still struggling to catch up. Both Dr Castro and Mr Muoi will have much to learn in Beijing this week.