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Thusfar we don't have a use case where the generality of discriminators is needed versus what can be done with the unparse-time equivalent of dfdl:choiceDispatchKey.
The situation is, in general, quite symmetric with parsing, so we should expect the same sorts of solutions: backtracking through the choice alternatives selecting the first one that doesn't cause an unparse error, use of specific asserts/discriminators for unparse time to control the backtracking, or an unparse version of choice resolution by dispatch.
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