docs: public void addValuesToKey(K key, Collection<V> valueCollection) {
          if (!valueCollection.isEmpty()) {
              // Create a new collection to store the values (will be changed to internal type by call
              // to putIfAbsent anyway)
              List<V> collectionToAppendValuesOn = new ArrayList<V>();
              List<V> existing = this.putIfAbsent(key, collectionToAppendValuesOn);
              if (existing != null) {
                  // Already had a collection, discard the new one and use existing
                  collectionToAppendValuesOn = existing;
              }
              collectionToAppendValuesOn.addAll(valueCollection);
          }
      }