Problem was that the OASIS writer was simply ignoring
all top level proxy cells (PCells, Library references).
The original bug #1835 fixed that by changing the
reader behavior, so it would keep top level proxies.
However, doing the spin through the writer got them
removed and in addition, the cleanup happing during
editing would also remove them.
Solution is to centralize the strategy of cleaning
cells. The cleanup now is changed to not remove
proxy top cells if they are the only ones. This is
consistent with the previous reader-only behavior.
The writer implements the same behavior by means
dropping cells marked for cleanup, instead of simply
skipping all proxy cells.
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