btree shrinking again

Hello,

I'm just taking the btree shrinking problem, and saw in the README:

+  Deletions are handled by getting a super-exclusive lock on the target  page, so that no other backend has a pin on
thepage when the deletion  starts.  This means no scan is pointing at the page.  This is OK for  deleting leaf items,
probablynot OK for deleting internal nodes;  will need to think harder when it's time to support index compaction.
 


In what cases is not OK to delete an item from an internal node, holding
a super-exclusive lock?  (I assume this means LockBufferForCleanup()).

-- 
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
"Los dioses no protegen a los insensatos.  Éstos reciben protección de
otros insensatos mejor dotados" (Luis Wu, Mundo Anillo)