Goal:
  
  G : t
  HI : invariants
  z : Z
  z0 : Z
  H : 0 <= z
  z1 : Z
  H1 : z < z0 /\ z1 = z0 \/ z0 <= z /\ z1 = z
  ============================
   (0 <= z1)


DEBUG:Going to execute:
zify_saturate