transformSortClause is passed the untransformed aggorder list, which is in fact a list of SortBy nodes, and it returns the transformed list (SortGroupClause nodes), which is stored back into the aggorder field a bit further down.
There are a number of regression tests that would fail in obvious ways if this code didn't work.
Right Tom. I got confused, because the comment at Aggref struct definition told that it is a list of SortGroupClause. May be you can update your comments there.
