Complete the partial correctness proof for the tabulation implementation.
Complete the partial correctness proof for the tabulation implementation.

This theorem states that every winner selected by the concrete implementation
is a possible winner as defined by the specification.

Fixes #3

Not (yet) proved is that every possible winner will be selected by the
implementation, depending on the choice of tiebreaking function.

cjerdonek/formal-rcv,cjerdonek/formal-rcv