schematronx (Xpath): how to verify the absence of an attribute?

Greetings,

I would like to create a rule where my test verifies the absence of an
attribute in schematron.

For instance this works fine,

<assert test="a[@attribute]">

but for a *not exist* attribute, 

<assert test="a[not(@attribute)]">

doesn't work (if I'm not wrong).

No, the negation by 'report' instead an 'assert' doesn't fits my needs.

thanks,

regards,
Pedro Salazar. 
-- 
PS
pedro-b-salazar_at_ptinovacao_dot_pt
PGP:0E129E31D803BC61