code: public void setDefaultRuleSet(String ruleSetName) {
          if (ruleSetName == null) {
              if (publicRuleSetNames.length > 0) {
                  defaultRuleSet = findRuleSet(publicRuleSetNames[0]);
              } else {
                  defaultRuleSet = null;
                  int n = ruleSets.length;
                  while (--n >= 0) {
                     String currentName = ruleSets[n].getName();
                     if (currentName.equals("%spellout-numbering") ||
                         currentName.equals("%digits-ordinal") ||
                         currentName.equals("%duration")) {
  
                         defaultRuleSet = ruleSets[n];
                         return;
                     }
                  }
  
                  n = ruleSets.length;
                  while (--n >= 0) {
                      if (ruleSets[n].isPublic()) {
                          defaultRuleSet = ruleSets[n];
                          break;
                      }
                  }
              }
          } else if (ruleSetName.startsWith("%%")) {
              throw new IllegalArgumentException("cannot use private rule set: " + ruleSetName);
          } else {
              defaultRuleSet = findRuleSet(ruleSetName);
          }
      }
docs: Override the default rule set to use.  If ruleSetName is null, reset
  to the initial default rule set.
  @param ruleSetName the name of the rule set, or null to reset the initial default.
  @throws IllegalArgumentException if ruleSetName is not the name of a public ruleset.