code: void add( K key, V value ) {
          List<V> rules = map.get( key );
          if( rules == null ) {
              rules = new ArrayList<>();
              map.put( key, rules );
          }
          rules.add( value );
      }
docs: Add a value to this map. If the map previously contained a mapping for the key, then there are two values now.
  @param key the key
  @param value the value