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


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