Breaking out this one issue for a separate discussion thread.

Consider the choice below. There is a choice of two different elements named raw1 and raw2, then a value element which is computed based on one or the other of the alternatives.

<choice>
       
       <element name="raw1" type="xs:string"
       dfdl:lengthKind='pattern'
         dfdl:lengthPattern="[^\x7F]{0,99}"
     dfdl:terminator="%DEL;"/>
       <element name="raw2" type="xs:string"
            dfdl:lengthKind="explicit"
            dfdl:length="100"/>
</choice>
<element name='value' type='xs:string'
     dfdl:inputValueCalc='{ if (fn:exists( ../raw1 ) then ../raw1 else ../raw2 }'/>

Now, given the statement below:

SMH: There is no way to make a choice branch fail when unparsing. (The only 'backtracking' when unparsing a choice is when the infoset contains no branch at all then the spec states that each branch is examined in turn until one is found that successfully applies defaults. But that's not really backtracking, as you can statically deduce the branch from the schema alone, so the 'default' branch to use can be computed up front).

So is there any way to fix this and make the above choice work?

This is the only thing I can think of, which ends up with entirely separate elements for parsing and unparsing.

I am not sure this works or should work.

<sequence>
<!--

raw1 won't exist on unparsing
-->

<!--
raw2 will only occur parsing if raw1 doesn't

and doesn't exist on unparsing

-->

<element name="outputRaw" type="xs:string"

dfdl:inputValueCalc='{ "" }'

dfdl:outputValueCalc="{ if (fn:string-length(../value) eq 100 then ../value else fn:concat(../value, '') }" />

</group>