static _intersection<T>(iter: IIterator<Array<T>>): Set<T> | void {
      // pop the first element of iter
      let first = iter.next();
      // first will be undefined if iter is empty
      if (!first) {
        return;
      }

      // "initialize" the intersection from first
      let isect = new Set(first);
      // reduce over the remaining elements of iter
      return reduce(
        iter,
        (isect, subarr) => {
          // filter out all elements not present in both isect and subarr,
          // accumulate result in new set
          return new Set(subarr.filter(x => isect.has(x)));
        },
        isect
      );
    }

 it('should invoke the reducer if the iterable has just two items with no initial value', () => {
   let data = [1, 2];
   let result = reduce(data, (a, x) => a + x);
   expect(result).to.equal(3);
 });

 let reduced = () => reduce(data, (a, x) => a + x);