Fair point, but there's no high level reget method with the same signature as the get method (in partic. with a callback) so it'd be painful to have to perform regets with do_write
The last argument is optional. If not supplied, get_from is the same as get. If supplied, the transfer will start at that offset. You can find out at which byte to start using stat on the local file. You'll need to make no other changes are needed.