func (t *Iterator) Next(dst interface{}) (string, error) {
	t.fetchMore()
	if t.err != nil {
		return "", t.err
	}

	var doc *pb.Document
	var exprs []*pb.Field
	switch {
	case len(t.listRes) != 0:
		doc = t.listRes[0]
		t.listRes = t.listRes[1:]
	case len(t.searchRes) != 0:
		doc = t.searchRes[0].Document
		exprs = t.searchRes[0].Expression
		t.searchCursor = t.searchRes[0].Cursor
		t.searchRes = t.searchRes[1:]
	default:
		return "", Done
	}
	if doc == nil {
		return "", errors.New("search: internal error: no document returned")
	}
	if !t.idsOnly && dst != nil {
		if err := loadDoc(dst, doc, exprs); err != nil {
			return "", err
		}
	}
	return doc.GetId(), nil
}
func (t *Iterator) Next(dst interface{}) (string, error) {
	t.fetchMore()
	if t.err != nil {
		return "", t.err
	}

	var doc *pb.Document
	var exprs []*pb.Field
	switch {
	case len(t.listRes) != 0:
		doc = t.listRes[0]
		t.listRes = t.listRes[1:]
	case len(t.searchRes) != 0:
		doc = t.searchRes[0].Document
		exprs = t.searchRes[0].Expression
		t.searchCursor = t.searchRes[0].Cursor
		t.searchRes = t.searchRes[1:]
	default:
		return "", Done
	}
	if doc == nil {
		return "", errors.New("search: internal error: no document returned")
	}
	if !t.idsOnly && dst != nil {
		if err := loadDoc(dst, doc, exprs); err != nil {
			return "", err
		}
	}
	return doc.GetId(), nil
}
// Next returns the ID of the next result. When there are no more results,
// Done is returned as the error.
//
// dst must be a non-nil struct pointer, implement the FieldLoadSaver
// interface, or be a nil interface value. If a non-nil dst is provided, it
// will be filled with the indexed fields. dst is ignored if this iterator was
// created with an IDsOnly option.