add missing syn in navigation attributes

src/main/jastadd/Navigation.jrag

@@ -2,7 +2,7 @@ aspect Navigation {
   inh PetriNet PnObject.petriNet();
   eq PetriNet.getChild().petriNet() = this;
 
-  java.util.Collection<Place> Transition.incomingPlaces() {
+  syn java.util.Collection<Place> Transition.incomingPlaces() {
     java.util.Set<Place> incomingPlaces = new java.util.HashSet<>();
     for (Arc incomingArc : getInArcList()) {
       incomingPlaces.add(incomingArc.getSource().asPlaceNode().place());
@@ -10,7 +10,7 @@ aspect Navigation {
     return incomingPlaces;
   }
 
-  java.util.Collection<Place> Transition.outgoingPlaces() {
+  syn java.util.Collection<Place> Transition.outgoingPlaces() {
     java.util.Set<Place> outgoingPlaces = new java.util.HashSet<>();
     for (Arc outgoingArc : getOutArcList()) {
       outgoingPlaces.add(outgoingArc.getTarget().asPlaceNode().place());