the arithmetic calculator exercise in GFA Basic

For a couple of reasons, I tried that arithmetic calculator exercise in GFA Basic (v3.6 for the Atari ST; why not?), and although I selected a somewhat different approach, I think it's really like a non-recursive version of the recursive Lisp version I posted before; with state in variables which persist for the whole loop. This is where side-effects can get mysterious, compared to the neater encapsulation of state in each recursive call's parameters.