This is not the code but the way, you should expand a postfix to infix::
5 x y - / x y + 3 ^ 7 / +
5 (x-y) / xy+ 3^ 7 / +
(5/(x-y)) xy+ 3^ 7 / +
(5/(x-y)) (x+y)3^ 7 / +
(5/(x-y)) ((x+y)^3) 7 / +
(5/(x-y)) (((x+y)^3) / 7) +
(5/(x-y)) + (((x+y)^3) / 7)
