docs: public boolean containsKey(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 false;
  		}
  		
  		return innerMap1.containsKey(secondKey);
  	}