We’ve already got an e-vote back-up system
The argument that it will aid in sorting out proportional vote preferences is a red herring: all we need to do, once the votes are reliably counted and adjudicated, is to feed the results into a computer, in public, and it will distribute preferences for us in a fraction of a second.
Anybody else can repeat the figures on their own computer, if they so wish. As far as a paper trail of accountability is concerned, we already have a paper trail it's called a voting slip, isn't it?
