Zero sums: the final cut
You have already done the heavy lifting in proving that x + y = 0 => y = 0. If you would like to take this result to its logical
conclusion, you could try this optional bonus problem.
Hints: Remember that you can turn the == into two conditionals.
Also it turns out that (A => B) & (A => C) => (A => B & C).
Where to go from here?
Some ideas: