function XUniqBy(f, xf) {
    this.xf = xf;
    this.f = f;
    this.set = new _Set();
  }
Inserts the sub-list into the list, at the specified `index`. _Note that this is not
destructive_: it returns a copy of the list with the changes.
<small>No lists have been harmed in the application of this function.</small>

@func
@memberOf R
@since v0.9.0
@category List
@sig Number -> [a] -> [a] -> [a]
@param {Number} index The position to insert the sub-list
@param {Array} elts The sub-list to insert into the Array
@param {Array} list The list to insert the sub-list into
@return {Array} A new Array with `elts` inserted starting at `index`.
@example

     R.insertAll(2, ['x','y','z'], [1,2,3,4]); //=> [1,2,'x','y','z',3,4]