fix RHDPDA refactoring

alib2data/src/automaton/PDA/RealTimeHeightDeterministicDPDA.cpp

@@ -113,7 +113,7 @@ bool RealTimeHeightDeterministicDPDA::addReturnTransition(State from, std::varia
 		throw AutomatonException("State \"" + (std::string) to.getName() + "\" doesn't exist.");
 	}
-	if (getPushdownStoreAlphabet().count(pop)) {
+	if (!getPushdownStoreAlphabet().count(pop)) {
 		throw AutomatonException("Pushdown store symbol \"" + (std::string) pop + "\" doesn't exist.");
 	}