But it doesn't help the library. It can't assume there are no
deletes just because the reader is read only. And if you know you don't
do deletes you can skip calling isdeleted readonly or not. The only way
this works cleanly is if you could ask if the "index" is readonly, not
the Reader. Knowing a Reader is read-only does not help a generic Lucene library from calling isDeleted - it still must be called in a generic library - only the specific app would know it could be skipped, because it would know the "index" is read-only, not just the transient Reader.