
use int.Int

let f (z: int) =
  let ref x1 = z in
  let ref x2 = 0 in
  let ref i = 0 in
  while i < 2 do
    variant { 2 - i }
    invariant { i > 0 -> x1 >= 0 }
    if i = 0 then
      x1 <- x1 + 1
    else
      x2 <- x2 + 1;
    i <- i + 1
  done

(*
counterexample: `x1 = -2`, `i = 0` at the beginning of the loop.

The execution of `f` with the counterexample:
- terminates normally if executed concretely;
- terminates in error if executed abstractly (invariant preservation
  fails)

It is a real counterexample due to underspecification (weak loop
invariant).
*)
