code: func NewDifferenceIterator(a, b NodeIterator) (NodeIterator, *int) {
  	a.Next(true)
  	it := &differenceIterator{
  		a: a,
  		b: b,
  	}
  	return it, &it.count
  }
docs: // NewDifferenceIterator constructs a NodeIterator that iterates over elements in b that
  // are not in a. Returns the iterator, and a pointer to an integer recording the number
  // of nodes seen.