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Well, it could apply to the packer too. for a file writer, you would
do an arbitrary # of of pack operations; each time the
data packed exceeded a certain size, a nice-sized chunk
would be written out to the underlying file. I'll have to think
about the actual subclassing interface a bit more.

regarding get_/set_position(), get_buffer(), and done() - these need to be
implemented when the unpacker is subclassed, but the implementation need not
be useful. I.e. you want to prevent the normal versions of these from being called,
but if you don't intend to support them, the implementation you provide could be
just a 'raise'.

And yes, 'read' makes sense since it does the same thing as file.read().

Things got busy in the last little while - new baby! but I'll take a look again
when I get a chance. I think the mods to the unpacker are pretty simple. And
I'll get the existing test code and look at that too.