Question on one of the Validation module constraint

Hi all,

(This is not a comment for the Candidate Draft).

I'm looking at possibly implementing the Validation module but I have a question with regard to one of the constraints:

In section "5.8.4.3 rule" we have:

[[
Exactly one of the following attributes:
- isPresent
- isNotPresent
- startsWith
- endsWith
- a custom rule defined by attributes from any namespace
is REQUIRED in any one <rule> element.
]]

I see how I can check for the presence of only one of the Validation attributes.
But how can I check that only a custom rule is specify?

It seems I need to check that only one or more custom attributes are present in the <rule>.
But can the custom attributes be in different namespaces or must they be in the same one?

I tend to think "any namespace" (singular) means they must be in the same namespace.
Is that correct?

Thanks,
-yves