fix immutable collection

Author: wadoon

key.core/src/main/java/de/uka/ilkd/key/rule/OneStepSimplifier.java

@@ -3,12 +3,7 @@
  * SPDX-License-Identifier: GPL-2.0-only */
 package de.uka.ilkd.key.rule;
 
-import java.util.ArrayList;
-import java.util.LinkedHashMap;
-import java.util.LinkedHashSet;
-import java.util.List;
-import java.util.Map;
-import java.util.Set;
+import java.util.*;
 
 import de.uka.ilkd.key.java.Services;
 import de.uka.ilkd.key.logic.JTerm;
@@ -124,8 +119,8 @@ private ImmutableList<Taclet> tacletsForRuleSet(Proof proof, String ruleSetName,
         ImmutableList<Taclet> result = ImmutableSLList.nil();
 
         // collect apps present in all open goals
-        Set<NoPosTacletApp> allApps =
-            proof.openGoals().head().ruleAppIndex().tacletIndex().allNoPosTacletApps();
+        Set<NoPosTacletApp> allApps = new HashSet<>(
+            proof.openGoals().head().ruleAppIndex().tacletIndex().allNoPosTacletApps());
         for (Goal goal : proof.openGoals().tail()) {
             allApps.retainAll(goal.ruleAppIndex().tacletIndex().allNoPosTacletApps());
         }