Maintain a set S of 9 elements, each with a counter.

Think of the following function:
f(x) =  counter(x),     if x in S
        - Sum_{y in S} counter(y),    if x not in S


You want to guarantee the following properties:

* when x is seen in the stream, f(x) increases by 9

* when some element other than x is seen, f(x) decreases by at most 1

Then, at the end, f(x) is positive for all frequent elements.
If f(x) > 0, it must be that x is in S.