Textual representation of listeners.

org.sat4j.pb/src/main/java/org/sat4j/pb/tools/VERIPBSearchListener.java

@@ -238,4 +238,8 @@ public class VERIPBSearchListener implements PBSearchListener<ISolverService> {
         this.conflict.append(" " + this.reason).append(" +");
     }
 
+    @Override
+    public String toString() {
+        return "VERIPB";
+    }
 }
\ No newline at end of file

org.sat4j.pb/src/main/java/org/sat4j/pb/tools/VoidPBTracing.java

@@ -9,4 +9,8 @@ public class VoidPBTracing extends PBSearchListenerAdapter<IPBSolverService> {
      */
     private static final long serialVersionUID = 1L;
 
+    @Override
+    public String toString() {
+        return "none";
+    }
 }