code: NFRuleSet findRuleSet(String name) throws IllegalArgumentException {
          NFRuleSet result = ruleSetsMap.get(name);
          if (result == null) {
              throw new IllegalArgumentException("No rule set named " + name);
          }
          return result;
      }
docs: Returns the named rule set.  Throws an IllegalArgumentException
  if this formatter doesn't have a rule set with that name.
  @param name The name of the desired rule set
  @return The rule set with that name