code: private static Collection<TileRef> getTiles(Map<Transition, Collection<TileRef>> transitions, Transition transition)
      {
          if (!transitions.containsKey(transition))
          {
              transitions.put(transition, new HashSet<TileRef>());
          }
          return transitions.get(transition);
      }
docs: Get the tiles for the transition. Create empty list if new transition.
  
  @param transitions The transitions data.
  @param transition The transition type.
  @return The associated tiles.