code: public int size(final K1 firstKey) {
  		// existence check on inner map
  		final HashMap<K2, HashMap<K3, V>> innerMap = map.get(firstKey);
  		if( innerMap == null ) {
  			return 0;
  		}
  		return innerMap.size();
  	}
docs: Returns the number of key-value mappings in this map for the second key.
  
  @param firstKey
  the first key
  @return Returns the number of key-value mappings in this map for the second key.