constraining a constraint placed inside a classSpec

Hi again,

While compiling schemas for a modification that I'm working on, I can 
see something like this:

  [xslt] WARNING: constraint for att.referring class does not have a 
context=. Resulting rule is applied to *all* elements.

Which is very true, because I have moved some constraintSpecs from an 
element spec (where, I gather, the element being defined supplies the 
context) into a spec containing a definition of a new attribute class 
that the element now belongs to.

I have searched the Guidelines for some examples, but failed to see any 
that would use "context=" with Schematron. A modified example that I 
earlier received from Syd has this:

<constraint>
   <sch:rule context="*[@referringMode eq 'pointer']"> ...

and it works really well, but is that the magic that I need for 
restricting Schematron to only the elements which are members of the 
class that I am defining?

A naive reading of the above context would suggest that the Schematron 
rule still applies to all elements except the root element (and then it 
of course filters them appropriately). So where is the trick, please, 
what am I missing?

Thanks in advance,

    Piotr