fixed serious bug in model generation from PRISM code; can't believe that didn't cause wrong models until now

Former-commit-id: 8fef881161

src/generator/PrismNextStateGenerator.cpp

@@ -336,9 +336,17 @@ namespace storm {
                                 for (auto const& stateProbabilityPair : *currentTargetStates) {
                                     // Compute the new state under the current update and add it to the set of new target states.
                                     CompressedState newTargetState = applyUpdate(stateProbabilityPair.first, update);
+                                    
+                                    // If the new state was already found as a successor state, update the probability
+                                    // and otherwise insert it.
+                                    auto targetStateIt = newTargetStates->find(newTargetState);
+                                    if (targetStateIt != newTargetStates->end()) {
+                                        targetStateIt->second += stateProbabilityPair.second * evaluator.asRational(update.getLikelihoodExpression());
+                                    } else {
                                         newTargetStates->emplace(newTargetState, stateProbabilityPair.second * evaluator.asRational(update.getLikelihoodExpression()));
                                     }
                                 }
+                            }
                             
                             // If there is one more command to come, shift the target states one time step back.
                             if (i < iteratorList.size() - 1) {