Schematron validation and uniqueness

Question

Hi, I'm trying to write some Schematron rules and one of them is supposed to check, if elements are unique in the scope of parent element. So I have an example xml structure:

<abc>
  <elem id="qw0">
    <a>1</a>
    <a>2</a>
    <a>3</a>
  </elem>
  <elem id="qw1">
    <a>1</a>
    <a>2</a>
    <a>3</a>
    <a>3</a>
  </elem>
</abc>

My rule should check if each of the element's "a" elements is unique. In this specific example, for elem with id="qw1" there are two elements "a" with value "3". This should not be allowed.

So far I've come to this kind of rule:

<iso:pattern id="doc.abc">
  <iso:title>checking ABC</iso:title>
  <iso:rule context="elem">
    <iso:assert test="count(a[. = current()]) = 1">TACs should be unique.</iso:assert>
  </iso:rule>
</iso:pattern>

But this does not work, as it kind of looks through the whole document, not just the direct children of elem.

Answer 1

I found out that this may be solved with the following rule:

<iso:pattern id="doc.abc">
  <iso:title>checking ABC</iso:title>
  <iso:rule context="a">
    <iso:assert test="count(parent::node()/a[. = current()) = 1">TACs should be unique.</iso:assert>
  </iso:rule>
</iso:pattern>

But this fires up the rule for every a element.

It would be more elegant to fire it for every elem, no a.