code: public void add(K key, V value) {
      if (treatCollectionsAsImmutable) {
        Collection<V> newC = cf.newCollection();
        Collection<V> c = map.get(key);
        if (c != null) {
          newC.addAll(c);
        }
        newC.add(value);
        map.put(key, newC); // replacing the old collection
      } else {
        Collection<V> c = map.get(key);
        if (c == null) {
          c = cf.newCollection();
          map.put(key, c);
        }
        c.add(value); // modifying the old collection
      }
    }
docs: Adds the value to the Collection mapped to by the key.