  We want to find a marked element out of a black box containing N elements.
When the number of marked elements is known this can be done elegantly with
Grover's algorithm, a variant of which even gives a correct result with
certainty. On the other hand, when the number of marked elements is not known
the problem becomes more difficult. For every prescribed success probability I
give an algorithm consisting of several runs of Grover's algorithm that matches
a recent bound by Buhrman and de Wolf on the order of the number of queries to
the black box. The improvement in the order over a previously known algorithm
is small and the number of queries can clearly still be reduced by a constant
factor.
