introduce PlaceholderNode

ui/src/main/java/edu/kit/iti/formal/psdbg/gui/controls/ScriptTree/PlaceholderNode.java

+package edu.kit.iti.formal.psdbg.gui.controls.ScriptTree;
+
+import de.uka.ilkd.key.proof.Node;
+import edu.kit.iti.formal.psdbg.gui.controls.TreeNode;
+
+public class PlaceholderNode extends AbstractTreeNode {
+    public PlaceholderNode(Node node) {
+        super(node);
+    }
+
+    @Override
+    public TreeNode toTreeNode() {
+        return  new TreeNode("-PlaceholderNode-", super.getNode());
+    }
+}

ui/src/main/resources/edu/kit/iti/formal/psdbg/examples/java/maxtriplet/script.kps

+script s0() {
+	symbex;
+	foreach{
+	    auto;
+	}
+}
+
 script s() {
 	symbex;
-}
\ No newline at end of file
+}
+