code: public Map<String, List<MiningField>> getMiningFields() {
          Map<String, List<MiningField>> miningFieldsMap = new HashMap<>();
          for (PMML4Model model : getModels()) {
              List<MiningField> miningFields = model.getRawMiningFields();
              miningFieldsMap.put(model.getModelId(), miningFields);
              model.getChildModels();
          }
          return miningFieldsMap;
      }
docs: Retrieves a Map with entries that consist of
  key -> a model identifier
  value -> the List of raw MiningField objects belonging to the model referenced by the key
  @return The Map of model identifiers and their corresponding list of raw MiningField objects