public ImportedMlFunction outputFunction(String outputName, String functionName) {
            RankingExpression outputExpression = owner().expressions().get(outputs.get(outputName));
            if (outputExpression == null)
                throw new IllegalArgumentException("Missing output '" + outputName + "' in " + this);
            return new ImportedMlFunction(functionName,
                                          new ArrayList<>(inputs.values()),
                                          outputExpression.getRoot().toString(),
                                          asStrings(inputMap()),
                                          Optional.empty());
        }
Returns the expression this output references as an imported function
model-integration/src/main/java/ai/vespa/rankingexpression/importer/ImportedModel.java