Goal:
    G : t  z : Z
  ============================
   (z <=? 0 -> exists z0 : Z, Some z = Some z0 /\ z0 <= 0)


DEBUG:Going to execute:
repeat
            match goal with
            | |- ?X1 -> ?X2 => intro
            | |- ~ _ => unfold not at 1; intro
            end