When writing variations of a schema with a sequence of two nillable elements, I came across a certain scenario that seems to consistently result in the following error:

Runtime Schema Definition Error: Unparser is not yet implemented.

[error] Schema context: element.doc7. Location line 140 column 6 in file:/tmp/text_nil_only45662042067256730168.dfdl.xsd. Data Context: byte 0.

[error]     at edu.illinois.ncsa.daffodil.processors.ParseOrUnparseState.SDE(PState.scala:153)

[error]     at edu.illinois.ncsa.daffodil.processors.unparsers.DummyUnparser.unparse(Unparser.scala:113)

[error]     at edu.illinois.ncsa.daffodil.processors.unparsers.Unparser.unparse1(Unparser.scala:57)

[error]     at edu.illinois.ncsa.daffodil.processors.unparsers.SeqCompUnparser$$anonfun$unparse$2.apply(Unparser.scala:100)

.....

This only occurs when one of the nillable elements in the sequence has the nilValue set by referencing a defined format, and the second one is set with a different method. If both of the elements have the nilVaue set via a referenced format, the test works without issues.

I have tests with the following scenarios (red indicates failure):

Note that the only failing scenarios are the cases where one nilValue is set via a referenced element and the second is set a different way, regardless of ordering.

See the tests in red above in the following files:
daffodil-test/src/test/resources/edu/illinois/ncsa/daffodil/section13/nillable/literal-value-nils-unparse.tdmldaffodil-test/src/test/scala-debug/edu/illinois/ncsa/daffodil/section13/nillable/TestNillableUnparseDebug.scala
daffodil-test/src/test/scala/edu/illinois/ncsa/daffodil/section13/nillable/TestNillableUnparse.scala