docs: private void initPrefixStatsField(String field) {
      if (!singlePositionPrefix.containsKey(field)) {
        singlePositionPrefix.put(field, new HashSet<String>());
      }
      if (!multiplePositionPrefix.containsKey(field)) {
        multiplePositionPrefix.put(field, new HashSet<String>());
      }
      if (!setPositionPrefix.containsKey(field)) {
        setPositionPrefix.put(field, new HashSet<String>());
      }
    }