Goal:
  
  z : Z
  z0 : Z
  z1 : Z
  H : (z <= z1)%Z
  H0 : (z0 <= z1)%Z
  z2 : Z
  H2 : (z < z0)%Z /\ z2 = z0 \/ (z0 <= z)%Z /\ z2 = z
  ============================
   (z2 <= z1)%Z


DEBUG:Going to execute:
zify_saturate