anchor.diff:      ..retreatTime = 0.2
       ..stayTime    = 2.4
       ..maxConcurrentNpc = 3
  -    ..npcSpawnProb     = 0.025;
  +    ..npcSpawnProb     = 0.04;
   
     LevelSpec lvl10 = new LevelSpec()
       ..columns     = 3
  
positive.code:     ..retreatTime = 0.2
      ..stayTime    = 2.6
      ..maxConcurrentNpc = 3
      ..npcSpawnProb     = 0.018;
  
    LevelSpec lvl9 = new LevelSpec()
      ..columns     = 3
  
negative.code:     ..spawnTime   = 1.2
      ..retreatTime = 0.2
      ..stayTime    = 2.2
      ..maxConcurrentNpc = 4
      ..npcSpawnProb     = 0.05;
  
    var levelSpecs = [ lvl1, lvl2, lvl3, lvl4, lvl5, lvl6, lvl7, lvl8, lvl9, lvl10 ];
  
  