code: public void addRuleSet (String prefix, TileSetRuleSet ruleset)
      {
          // configure the ruleset with the appropriate prefix
          ruleset.setPrefix(prefix);
  
          // and have it set itself up with the digester
          _digester.addRuleSet(ruleset);
  
          // add a set next rule which will put tilesets with this prefix
          // into the array list that'll be on the top of the stack
          _digester.addRule(ruleset.getPath(),
                            new ValidatedSetNextRule("add", Object.class,
                                                     ruleset));
      }
docs: Adds a ruleset to be used when parsing tiles. This should be an
  instance of a class derived from {@link TileSetRuleSet}. The prefix
  will be used to configure the ruleset so that it matches elements
  at a particular point in the XML hierarchy. For example:
  
  <pre>
  _parser.addRuleSet("tilesets", new UniformTileSetRuleSet());
  </pre>