Checking if a Block can be Purged

Can anyone tell me how to establish whether a block (in an AutoCAD14 drawing) is available for purging using AutoLisp?
I can use 'tblsearch' to check the block exists in the drawing database, and I can use 'ssget' to establish if the block is inserted directly into the drawing.
(if (and (tblsearch "BLOCK" bname) (not (ssget "X" (cons '(0 . "INSERT") (list (cons 2 bname))))) ) (command "PURGE" "B" bname) )
Unfortunately the lisp routine fails if the block is inserted into the drawing as a nested (embedded) block because 'ssget' doesn't find it.
Basically, I need to know whether the block is inserted into the drawing either directly, or indirectly as a nested (embedded) block. Is there an entity code or similar that will tell me this?
TIA,
--
Razoo



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