accumulation: Fix reward construction UNTESTED

prism/src/explicit/AccumulationTransformation.java

@@ -116,11 +116,8 @@ public class AccumulationTransformation<M extends ModelExplicit> implements Mode
 		for (int i=0; i < accexp.getFunctions().size(); i++) {
 			Object rewardIndex = accexp.getFunctions().get(i).getRewardIndex();
 			
-			RewardStruct rewStruct = ExpressionReward.getRewardStructByIndexObject(rewardIndex, mc.modulesFile, originalModel.getConstantValues());
-			ConstructRewards constructRewards = new ConstructRewards();
-			constructRewards.allowNegativeRewards();
-			
-			Rewards reward = constructRewards.buildRewardStructure(originalModel, rewStruct, mc.getConstantValues());
+			int rewStructIndex = ExpressionReward.getRewardStructIndexByIndexObject(rewardIndex, mc.modulesFile, originalModel.getConstantValues());
+			Rewards reward = ((ProbModelChecker)mc).constructRewards(originalModel, rewStructIndex, true /* allow negative rewards */);
 			rewards.put(rewardIndex,reward);
 		}