docs: public void put(K key, V value) {
          checkNotNull(key, "Key cannot be null");
          checkNotNull(value, "Value cannot be null");
  
          Set<V> values = backingMap.get(key);
          if (values == null) {
              values = new HashSet<V>();
              backingMap.put(key, values);
          }
          values.add(value);
      }