public Tier GetPreviousTier(Tier tier)
 {
     for (int i = 1; i < Tiers.Count; i++)
         if (Tiers[i] == tier)
             return Tiers[i - 1];
     return null;
 }
 public bool PurchaseTier(Tier tier)
 {
     if (GoldCoins >= tier.Cost)
     {
         GoldCoins -= tier.Cost;
         AvailableTiers.Add(tier.Name);
         Save();
         return true;
     }
     else
         return false;
 }
 public Tier GetNextTier(Tier tier)
 {
     for (int i = 0; i < Tiers.Count - 1; i++)
         if (Tiers[i] == tier)
             return Tiers[i + 1];
     return null;
 }