anchor.diff:      ..retreatTime = 0.2
       ..stayTime    = 2.6
       ..maxConcurrentNpc = 3
  -    ..npcSpawnProb     = 0.018;
  +    ..npcSpawnProb     = 0.03;
   
     LevelSpec lvl9 = new LevelSpec()
       ..columns     = 3
  
positive.code:     ..retreatTime = 0.2
      ..stayTime    = 2.4
      ..maxConcurrentNpc = 3
      ..npcSpawnProb     = 0.025;
  
    LevelSpec lvl10 = new LevelSpec()
      ..columns     = 3
  
negative.code:     ..retreatTime = 0.2
      ..stayTime    = 3.0
      ..maxConcurrentNpc = 2
      ..npcSpawnProb     = 0.02;
  
    LevelSpec lvl7 = new LevelSpec()
      ..columns     = 3
  