public IntermediateOperation input() {
            return operation.inputs().get(inputNumber);
        }
An operation and an input number which we may want to insert a rename operation at.
That is, we may want to change op(..., input, ...) to op(..., rename(input), ...).

This class is (and must remain) immutable.
model-integration/src/main/java/ai/vespa/rankingexpression/importer/DimensionRenamer.java