code: public static Map<String, String> convertPropertyConnectionsToSimpleForm(Map<String, Set<String>> connections) {
          Map<String, String> result = new HashMap<String, String>();
          if (connections == null) {
              return result;
          }
          for (Map.Entry<String, Set<String>> entry : connections.entrySet()) {
              result.put(entry.getKey(), Joiner.on(',').join(entry.getValue()));
          }
          return result;
      }
docs: Converts a complex property connection map (key:String, value:Set of Strings) to a simple property connection
  map, so that the connections can easily be saved to the edges in the model graph.