Parsing expressions

[xset]

Hello

I wanted to explode expression like X^2 + 5 to the list of {operation,firstOP,secondOP}, i found i can do it in CAS window with "at" special syntax like 'X^2+5'[1 +-].

It rewrites it as at('X^2+5',0) and gives answer _x -> _x (looks like lambda).
'X^2+5'[2 +-] gives x^2
'X^2+5'[3 +-] gives 5

There is no really 'at' function and this code does not work in program.

So, question is: how to parse expressions ? If there some command like OBJ-> from 50g ?

Many many thanks anyway!
XSet