# Hint

If the first number is 0, the task is easy: the solution is
simply the second number. If the first number is not 0, strip a
succ-functor off the first number and add the result to the
second number. Then add the succ-functor again. In other words,
if adding `X`

to `Y`

is `Z`

,
then adding `succ(X)`

to `Y`

is
`succ(Z)`

.