peano_answer(two_plus_three, s(s(s(s(s(z)))))).
peano_answer(two_times_three, s(s(s(s(s(s(z))))))).
peano_answer(factorial_four, s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(s(z))))))))))))))))))))))))).