TOS='-nil-' stmt = 'push 3'
TOS='3' stmt = 'push 5'
TOS='5' stmt = 'add'
TOS='3+5' stmt = 'push apples'
TOS='apples' stmt = 'add'
TOS='3+5+apples' stmt = 'push oranges'
TOS='oranges' stmt = 'push 9'
TOS='9' stmt = 'push bananas'
TOS='bananas' stmt = 'push peels'
TOS='peels' stmt = 'mul'
TOS='bananas*peels' stmt = 'add'
TOS='9+bananas*peels' stmt = 'add'
TOS='oranges+9+bananas*peels' stmt = 'mul'
TOS='(3+5+apples)*(oranges+9+bananas*peels)' stmt = 'print'
out: '(3+5+apples)*(oranges+9+bananas*peels)'