incorrect unique constraint in Primer?

I have a question about the example of a unique constraint in the primer 
in section 5.1.  The following example is given:

  <unique name="dummy1">
   <selector xpath="r:regions/r:zip"/>
   <field    xpath="@code"/>
   <field    xpath="r:part/@number"/>
  </unique>

However, since a zip element may contain more than 1 parts the path 
expression "r:part/@number" does not result always in a single node, 
which makes this an illegal unique constraint (cfr. section 3.11.1 in 
XML Schema Part 1: Structures).

Actually I very strongly suspect that the constraint that they are 
trying to express is actually not expressible as a unique constraint.

Is this correct or have I misunderstood something?

-- 
    Jan Hidders

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Received on Monday, 20 October 2003 12:22:36 UTC