code: public boolean containsKey(final K1 firstKey, final K2 secondKey) {
  		// existence check on inner map
  		final HashMap<K2, V> innerMap = map.get(firstKey);
  		if( innerMap == null ) {
  			return false;
  		}
  		return innerMap.containsKey(secondKey);
  	}
docs: Existence check of a value (or <tt>null</tt>) mapped to the keys.
  
  @param firstKey
  first key
  @param secondKey
  second key
  @return true when an element (or <tt>null</tt>) has been stored with the keys