docs: function(parallels = 1) {
      this._parallels = parallels || 1;
  
      /**
       * _queueCounter
       * each lock() increased this number
       * each unlock() decreases this number
       * If _qC==0, the state is in idle
       * @type {Number}
       */
      this._qC = 0;
  
      /**
       * _idleCalls
       * contains all promises that where added via requestIdlePromise()
       * and not have been resolved
       * @type {Set<Promise>} _iC with oldest promise first
       */
      this._iC = new Set();
  
      /**
       * _lastHandleNumber
       * @type {Number}
       */
      this._lHN = 0;
  
      /**
       * _handlePromiseMap
       * Contains the handleNumber on the left
       * And the assigned promise on the right.
       * This is stored so you can use cancelIdleCallback(handleNumber)
       * to stop executing the callback.
       * @type {Map<Number><Promise>}
       */
      this._hPM = new Map();
      this._pHM = new Map(); // _promiseHandleMap
  }