code: function (np, grs, adv, numGroups) {
    // the group size in here refers to the maximal reduced group size
    if (np < 2) {
      return 'needs at least 2 players';
    }
    if (grs < 2) {
      return 'group size must be at least 2';
    }
    if (adv >= grs) {
      return 'must advance less than the group size';
    }
    var isUnfilled = (np % numGroups) > 0;
    if (isUnfilled && adv >= grs - 1) {
      return 'must advance less than the smallest match size';
    }
    if (adv <= 0) {
      return 'must eliminate players each match';
    }
    return null;
  }
docs: ------------------------------------------------------------------ Invalid helpers ------------------------------------------------------------------