Re: Are there invariants in XML processing?

For a Schematron introduction to this, including *document* invariants
(fixed and co-occurrence) and* input/output* invariants, and
*round-trip *invariants,
see Six Kinds of Validation using Schematron at
https://www.schematron.com/document/279.html

For people interest in the practical theory of invariants, Bertam Meyers'
work on Design by Contact still is a great place to start: he couches it in
functional terms of pre-condition, invariant,  post-condition, which I
think is more systematic than just "invariants".

For the example, the Schematron IO validation (output to input) might be
(roughly)

<sch:pattern id="publicMilitaryIndicator-nullTransform"
documents="'input.xml'">

     <sch:rule context="publicMilitaryIndicator">
             <sch:let name="matching-output"
value="myFunc:findMatchingRow(., 'output.xml')" />

              <sch:assert test="TYPE = 'A' or TYPE = 'B' or TYPE = 'C'"
role="pre-condition" >Every publicMilitaryIndicator input should have a
TYPE  with value A, b, or C (i.e., Civil, Joint, Military)</sch:assert>

               <sch:assert test="$matching-output/TYPE = 'A' or
$matching-output/TYPE = 'B' or $matching-output/TYPE = 'C'"
role="post-condition" >Every publicMilitaryIndicator output should have a
TYPE  with value A, b, or C (i.e., Civil, Joint, Military)</sch:assert>

             <sch:assert test="$matching-output" role="invariant">Every
input publicMilitaryIndicator should have a matching output</sch:assert>

             <sch:assert test="$matching-output/TYPE= ./TYPE"
role="invariant" >Every input publicMilitaryIndicator TYPE value should be
carried through to the output</sch:assert>

</sch:rule></sch:pattern>

Rick