private Card chooseCardOnStack(SpellAbility sa, Player ai, Game game) {
    for (SpellAbilityStackInstance si : game.getStack()) {
      final Card source = si.getSourceCard();
      final SpellAbility abilityOnStack = si.getSpellAbility(true);

      if (sa.hasParam("Choices")
          && !abilityOnStack.getHostCard().isValid(sa.getParam("Choices"), ai, sa.getHostCard())) {
        continue;
      }
      final ApiType threatApi = abilityOnStack.getApi();
      if (threatApi != ApiType.DealDamage && threatApi != ApiType.DamageAll) {
        continue;
      }

      List<? extends GameObject> objects = getTargets(abilityOnStack);

      if (!abilityOnStack.usesTargeting()
          && !abilityOnStack.hasParam("Defined")
          && abilityOnStack.hasParam("ValidPlayers"))
        objects =
            AbilityUtils.getDefinedPlayers(
                source, abilityOnStack.getParam("ValidPlayers"), abilityOnStack);

      if (!objects.contains(ai) || abilityOnStack.hasParam("NoPrevention")) {
        continue;
      }
      int dmg =
          AbilityUtils.calculateAmount(source, abilityOnStack.getParam("NumDmg"), abilityOnStack);
      if (ComputerUtilCombat.predictDamageTo(ai, dmg, source, false) <= 0) {
        continue;
      }
      return source;
    }
    return null;
  }
  @Override
  public PaymentDecision visit(CostExileFromStack cost) {

    Integer c = cost.convertAmount();
    if (c == null) {
      final String sVar = ability.getSVar(cost.getAmount());
      // Generalize cost
      if (sVar.equals("XChoice")) {
        return null;
      }
      c = AbilityUtils.calculateAmount(source, cost.getAmount(), ability);
    }
    List<SpellAbility> chosen = new ArrayList<SpellAbility>();
    for (SpellAbilityStackInstance si : source.getGame().getStack()) {
      SpellAbility sp = si.getSpellAbility().getRootAbility();
      if (si.getSourceCard().isValid(cost.getType().split(";"), source.getController(), source)) {
        chosen.add(sp);
      }
    }
    return chosen.isEmpty() ? null : PaymentDecision.spellabilities(chosen);
  }