ttt: do not widen visibility of internal method

[skip CI]

Author: mtf90

algorithms/active/ttt/src/main/java/de/learnlib/algorithms/ttt/mealy/TTTLearnerMealy.java

@@ -61,7 +61,7 @@ protected TTTTransition<I, Word<O>> createTransition(TTTState<I, Word<O>> state,
 
     @Override
     @SuppressWarnings("unchecked")
-    public boolean refineHypothesisSingle(DefaultQuery<I, Word<O>> ceQuery) {
+    protected boolean refineHypothesisSingle(DefaultQuery<I, Word<O>> ceQuery) {
         DefaultQuery<I, Word<O>> shortenedCeQuery =
                 MealyUtil.shortenCounterExample((TTTHypothesisMealy<I, O>) hypothesis, ceQuery);
         return shortenedCeQuery != null && super.refineHypothesisSingle(shortenedCeQuery);