@Override
    public SimpleDocumentFieldValues setOutputValue(String fieldName, FieldValue fieldValue, Expression exp) {
        Field field = output.getField(fieldName);
        if (field == null) {
            throw new IllegalArgumentException("Field '" + fieldName + "' not found in document type '" +
                                               output.getDataType().getName());
        }
        output.setFieldValue(field, fieldValue);
        return this;
    }
Creates an instance with empty output
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/SimpleDocumentFieldValues.java