Goal:
  
  G : t
  HI : invariants
  z : Z
  H : 0 <= z
  z0 : Z
  H0 : z < 0 /\ z0 = 0 \/ 0 <= z /\ z0 = z
  ============================
   (z = z0)


DEBUG:Going to execute:
Zify.zify_post_hook