code: public Object set(final K1 firstKey, final K2 secondKey, final V value) {
  		// existence check on inner map
  		HashMap<K2, V> innerMap = map.get(firstKey);
  		
  		if( innerMap == null ) {
  			// no inner map, create it
  			innerMap = new HashMap<K2, V>();
  			map.put(firstKey, innerMap);
  		}
  		
  		return innerMap.put(secondKey, value);
  	}
docs: Insert a value
  
  @param firstKey
  first key
  @param secondKey
  second key
  @param value
  the value to be inserted. <tt>null</tt> may be inserted as well.
  @return null or the value the insert is replacing.