[DFDL-WG] clarify array expressions

[Mike Beckerle]

Some folks are trying to create a DFDL model for IMF (internet message
format ... basically email)

There is this header structure which really wants unordered sequences.
Since we don't have those yet
I told them to use an array of choice like this:

<xs:element name="hdrArray" minOccurs="1" maxOccurs="25">
   <xs:complexType>
     <xs:choice>
        <xs:element ref="TO_HDR"/>
        <xs:element ref="FROM_HDR" />
     </xs:choice>
     ...

But then there are these contraints, like there should be only one FROM
address, but there can be several TO addresses.

So,  one might want to write a DFDL expression using an XPath-like query
path in it. E.g.,

<xs:assert>{ dfdl:occursCount( hdrArray/FROM_HDR ) eq 1 }</xs:assert>


Here's the question: Is the DFDL expression in this assert legal?

I advised them that no, it is not, because hdrArray is an array and isn't
being indexed, while FROM_HDR is not an array, but is being used as the
argument to dfdl:occursCount.

I think DFDL's expression language is simply not powerful enough to express
these constraints.

I'm just looking for affirmation or discussion if there is more to say
here.
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