code: public LoggingPreferences addPreferences(LoggingPreferences prefs) {
      if (prefs == null) {
        return this;
      }
      for (String logType : prefs.getEnabledLogTypes()) {
        enable(logType, prefs.getLevel(logType));
      }
      return this;
    }
docs: Adds the given logging preferences giving them precedence over existing
  preferences.
  
  @param prefs The logging preferences to add.
  @return A references to this object.