Lars Huttar wrote:

>count(
>  ancestor-or-self::A except ancestor-or-self::B[1]/ancestor::A
>     )

Thanks Lars, this looks way better than what I came up with, especially
it handles the case "there is no ancestor B" much more nicely than my
document-node()-kludge. 'except' is really powerful - unfortunately I
often forget about it...

(David has come up with essentially the same expression - thanks as well.)

-Christian