added a condition to force backtrack

integraal/integraal-explanation/src/main/java/fr/boreal/explanation/solving_enumerating/whyprovenance/UnambiguousProofTreeWhyProvenanceSolver.java

@@ -120,8 +120,8 @@ public class UnambiguousProofTreeWhyProvenanceSolver implements Solver {
                 }
 
                 if(foundImprovableAtom){
-                    // see how to hanlde properly lastOperation = Operation.BACKTRACK;
-                    //continue;
+                    //we force backtrack
+                    atomsToProveQueue.clear();
                 }
                 atomsToProveQueue.sort(Comparator.comparingInt(a -> alternativesFor.get(a).size()));