docs: public int size(final K1 firstKey, final K2 secondKey) {
  		// existence check on inner map
  		final HashMap<K2, HashMap<K3, V>> innerMap1 = map.get(firstKey);
  		if( innerMap1 == null ) {
  			return 0;
  		}
  		
  		// existence check on inner map1
  		final HashMap<K3, V> innerMap2 = innerMap1.get(secondKey);
  		if( innerMap2 == null ) {
  			return 0;
  		}
  		return innerMap2.size();
  	}