XPath or xsl to count a number of distinct values of a

Hi.
Hopefully a very quick question:
I have an attribute such as @class, which may have a value "h1", "H2" etc.
I want to calculate how many distinct values are there so that "H2"
and "h2" are counted as two occurrences, but following occurrences of
either do not increment a counter.
Basically a quality metric for a consistency of a document.

Is it possible with Xpath alone? (sth like count(distinct-values(//@class)) ?)

Lech