datasource and count()

Hello,
I wrote a little datasource that i need to browse the filesystem..
find('all'), find('first') works without any problem, but I tried to
use find('count') and I discovered that I need to implement
calculate() on my datasource..
but calculate documentation says that "Returns an SQL calculation,
i.e. COUNT() or MAX()" !?
find(count) can be used only by SQL based datasource??

or I am missing something?
thank you

PS: it would be cool add some information about this in the cookbook too..

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pietro