“…The first result of this type was given by Siegmund [34]. The basic idea was subsequently investigated in many contexts, and a small selection of the literally hundreds of papers on the topics is [ [1,2,3,7,8,9], [11,12,15,17,18,20,24,25,29,30,33]]. Necessary and sufficient conditions under which a prescribed scheme is asymptotically optimal are discussed in [10,31,32], while [21] gives a survey of rare-event simulation.…”