docs: public void removeMapping(K key, V value) {
      if (treatCollectionsAsImmutable) {
        Collection<V> c = map.get(key);
        if (c != null) {
          Collection<V> newC = cf.newCollection();
          newC.addAll(c);
          newC.remove(value);
          map.put(key, newC);
        }
  
      } else {
        Collection<V> c = get(key);
        c.remove(value);
      }
    }