code: function instance(defaults, options) {
    if (!singletonQS) {
      singletonQS = queryState(defaults, options);
    } else if (defaults) {
      singletonQS.setIfEmpty(defaults);
    }
  
    return singletonQS;
  }
docs: Returns singleton instance of the query state.
  
  @param {Object} defaults - if present, then it is passed to the current instance
  of the query state. Defaults are applied only if they were not present before.