Check that the environment ID is not NIL
and does not pertain to a primitive item.
Extract the Nth item.
(Int) = NBS_FIND_NTH_ITEM (ENVID,POSN,ID,STATUS)
ENVID = INTEGER (Given)
Identifier of the parent of the item which is to be found.
POSN = INTEGER (Given)
Number of item to find (first item is item #1).
ID = INTEGER (Returned)
Identifier of the found item (zero if not found).
STATUS = INTEGER (Given and returned)
global status. Possible return values are,
|Parent is primitive|
|No item of this name exists|