const bound: int = 1000;

node counters(z: bool) returns (eq_nind : bool ; probe x, probe y : int)
var
  b1, b2 : bool;
  a, b : bool;
let

  a = false -> (if pre a then pre x < bound else pre x <= 0);
  --a = false -> pre(if a then x < bound else x <= 0);
  x = bound -> (if a then pre x + 1 else pre x - 1);

  b = true -> (if pre b then pre y < bound else pre y <= 0);
  --b = true -> pre(if b then y < bound else y <= 0);
  y = 0 -> (if b then pre y + 1 else pre y - 1);

  b1 = x=bound;
  b2 = y=0;


  guarantee xy : x+y = bound;
  guarantee bb : b1 = b2;
  guarantee rp : -bound <= (x-y) and (x-y) <= bound;

tel