 Complete the following phrase:  Since 1927 the World Snooker Championship had been played as a single-elimination tournament, but between 1964 and 1968, it was defended over seven challenge matches. Following a hiatus after the 1957 World Professional Match-play Championship, the event was revived by Rex Williams on a challenge basis, with the champion being opposed by prominent players. This began in 1964, organised by the Billiards Association and Control Council. The 1957 champion John Pulman contested and won all seven challenge matches against various opponents in the next five years, until the tournament reverted to a knock-out format in 