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Comment: Added item about global elements and simple types with IVC/OVC

(Short URL for this page is https://s.apache.org/daffodil-experience-with-computed-elements)

Daffodil is the first DFDL implementation to provide the advanced features of

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It is essential that when unparsing it is allowed for the infoset to already contain output-computed elements. The values of these will be discarded and recomputed during unparsing, but they have to be tolerated if present, and created if not present.

Allow IVC/OVC on Global Elements, SimpleTypes

The restriction that IVC/OVC properties are not allowed on global element is arbitrary. It was probably put in place with an eye towards reducing complexity of implementations. It is, however, unnecessary and gets in the way of some useful cases.  This restriction should be removed.

Similarly, IVC/OVC are not allowed to be declared on simple types. This restriction is also unnecessary and makes reuse harder.

SDE for element in hidden group that has no default, nor dfdl:outputValueCalc

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