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Hmm.  I suspect the flush docs() are too strong (does flush 
really promise to materialize bytes *on disk*?  it doesn't for 
other Python file objects, you also need os.fsync() for that).

Your point is well taken, though, and whatever flush() does 
normally do, it's not going to do it for a copy-on-write mmap.  
So fine by me if we declare that attempting to flush() a copy-
on-write mmap raises an exception.