I was trying to default a child dataset to empty in a record structure.
My first guess at the syntax was, e.g.:
Discussed with Gavin Halliday - and DEFAULT was suggested, but I couldn't get that to work in this context, but DATASET(cred) kids := DATASET(, r); did work.
It would be good to support the abbreviated form using 
and I think Gavin thought some of the forms with DEFAULT should also have worked.