code: public Object realizeNas(int index, Object value) {
      for (int i = values.size(); i < index; i++)
        values.add(ExpressionInfo.VALUE_NOT_AVAILABLE);
      return value;
    }
  
    public List<Object> getValues() {
      return new ArrayList<>(values);
    }
  
    public Integer getCurrentRecordingVarNum() {
      if (startedRecordings.isEmpty()) {
        return null;
      } else {
        return startedRecordings.peek();
      }
    }
  }
docs: Materializes N/A values without recording a new value.