code: function byLevel (a, b) {
      if (no(cachedLevelOf[a])) cachedLevelOf[a] = computeLevelOf(a)
  
      if (no(cachedLevelOf[b])) cachedLevelOf[b] = computeLevelOf(b)
  
      return cachedLevelOf[a] - cachedLevelOf[b]
    }
docs: Sorts tasks by their level.