code: function Set(arr) {
    Object.defineProperty(this, '__arr__', {value: []});
  
    if (arr) {
      arr.forEach(function (el) { this.push(el); }, this);
    }
  }
docs: new Set(arr)
  - arr (Array): Initial array to fill-in
  
  Creates new instance of Set.
  
  var s = new Set([1, 2, 1, 3]);
  s.toArray();
  // -> [1, 2, 3]