Object -> genies_lamp "brass lamp"
  with name "brass" "lamp",
       before
       [; Rub: if (self hasnt general) give self general;
               else give self ~general;
              "A genie appears from the lamp, declaring:^^
               ~Mischief is my sole delight:^
               If white means black, black means white!~^^
               She vanishes away with a vulgar wink.";
       ];
Object -> white_stone "white stone" with name "white" "stone";
Object -> black_stone "black stone" with name "black" "stone";
...
[ BeforeParsing;
   if (genies_lamp hasnt general) return;
   for (wn=1::)
   {   switch(NextWordStopped())
       {   'white': parse->(wn*2-3) = 'black';
           'black': parse->(wn*2-3) = 'white';
           -1: return;
       }
   }
];