DFDL Extension: Enum and Range table lookups

Some schemas have large simple type unions that combine enumerations with many members, and numeric ranges.

Translation between numeric values, and symbolic values (strings), is desirable.

A proposal for how to do this is:
https://cwiki.apache.org/confluence/display/DAFFODIL/Proposal%3A+Features+to+Support+Table-Lookup

(This one is now obsolete:
https://opensource.ncsa.illinois.edu/confluence/display/DFDL/Enumerations+and+Range+Tables+via+Simple+Type+Unions)

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Under review by the DFDL Workgroup, it was observed that these features require validation processing, and that was considered problematic given that validation is an optional feature of DFDL.�� However, the dfdl:checkConstraints function is not optional - excepting that the whole expression language is optional. So it is reasonable to add these features to an implementation that has either validation or dfdl:checkConstraints().

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