Parse zero or more reamaining generic associations.
(parse-generic-associations-rest prev-genassocs prev-span parstate) → (mv erp genassocs span new-parstate)
This is called after parsing the first generic association of a generic selection, which is required (i.e. there must be at least one). Thus, each generic association to parse (if any), is preceded by a comma. We stop when there is no more comma.
We pass to this function the list of generic expressions parsed so far, along with their span. This makes it easier to handle the span calculation.