code: public Collection<String> getIdentifiers() {
      List<String> keys = new LinkedList<String>();
      keys.addAll(this.stateMap.keySet());
      return keys;
    }
docs: Returns a collection containing the same Identifier Strings as this WorldState
  object. Modifications to the returned Collection do not impact this
  WorldState.
  
  @return a collection containing the same Identifier Strings as this WorldState.