Custom function COUNTIFUNIQUES in Excel

Question

Possible Duplicate:
Excel: how to count number of distinct values in a range?

I would like to create a custom function COUNTIFUNIQUES that extends the functionality of COUNTIFS.

Here is an example of how it would work:

     A1  B1
A1   A   1
A2   A   1
A3   A   2
A4   B   Y
A5   B   Z
A6   B   Z
A7   C   4
A8   C   5
A9   C   6

Here is the existing functionality:
=COUNTIFS(criteria_range[1], criteria[1], ...)

=COUNTIFS(A1:B9,"A") —> 3
=COUNTIFS(A1:B9,"B") —> 3

I think the desired functionality requires two improvements:

First: change COUNTIFS from *COUNTIFS(criteria_range[1], criteria[1], ...)* to
*COUNTIFS(count_range, criteria_range[1], criteria[1], ...)*


Second: have the function return unique result:

=COUNTIFUNIQUES(B1:B9,A1:A9,"A") —> 2
=COUNTIFUNIQUES(B1:B9,A1:A9,"B") —> 2

How would you implement this in VBA?

What I'm looking for is something in the form:

Function COUNTIFUNIQUES(...)

....

End Function

I don't believe any existing solution solves this problem "cleanly", if at all.