The author gratefully acknowledges the use of subroutines written by Joseph Lehár (Cambridge), Keith Horne (STScI) and Tom Marsh (Oxford), as well as those written by Press and Rybicki (1989). I also thank the bug-finding guinea-pigs Martin Still (Sussex), Mark O’Dell (Sussex), Erik Kuulkers (Amsterdam) and Thomas Augusteijn (Amsterdam) for their many useful comments and suggestions. Finally, I am indebted to Christian Knigge (Oxford) for his help with bug fixes and his many suggestions regarding significance estimates.