ghalliday/HPCC-Platform — comment on commit 11427df1a2e3a47ebdbbec7a7b9ba1fab57d899c
File: ecl/hql/hqlgram.y line 8546
By ghalliday:

Because the optional production requires a leading ','. The production above handles the case where it isn't supplied.
It needs to be separate productions, rather than using optSortList because of the clashes between the two preceeding productions