code: public List<StorageTier> getTiersBelow(String tierAlias) {
      int ordinal = getTier(tierAlias).getTierOrdinal();
      return mTiers.subList(ordinal + 1, mTiers.size());
    }
docs: Gets the list of {@link StorageTier} below the tier with the given tierAlias. Throws an
  {@link IllegalArgumentException} if the tierAlias is not found.
  
  @param tierAlias the alias of a tier
  @return the list of {@link StorageTier}