private static List<GroupingExpression> asList(GroupingExpression exp, List<? extends BucketValue> args) {
        List<GroupingExpression> ret = new LinkedList<>();
        ret.add(exp);
        ret.addAll(args);
        return ret;
    }
Returns the bucket at the given index.

@param i The index of the bucket to return.
@return The bucket at the given index.
@throws IndexOutOfBoundsException If the index is out of range.
container-search/src/main/java/com/yahoo/search/grouping/request/PredefinedFunction.java