Examples below:

1."+ - 5 4 3" should give
IntegerV 4
((5-4)+3=4)

2."if < - 5 4 2 then - 3 2 else + 9 10" should give
IntegerV 1
(we allow logic calculation)

3."@ @ \\x.\\y.@ x y \\z.+ z 1 4" should give
IntegerV 5
(this equals to (λx.(λy.x y))(λz.z+1)4 = 5 )