how to match the absence of a particular attribute

Hi guys,

how can I test the presence of a particular attribute of a node in an
"if" or "choose" tag?
I need to drive the behavior of the translation in two different ways,
if an attribute exists or not. I suppose the key point is the
capability to match the absence of an attribute, but I have found no
way to do this.

Thank you very much for any help.

Livio