docs: public static <K, V> List<V> addToValueList(Map<K, List<V>> map, K key, V value) {
          List<V> valueList = map.get(key);
          if (valueList == null) {
              valueList = new ArrayList<V>();
              map.put(key, valueList);
          }
          valueList.add(value);
  
          return valueList;
      }