Goal:
    z : Z  z0 : Z  z1 : Z  z2 : Z
  ============================
   ((z <= z0)%Z -> (z1 <= z2)%Z -> (z + z1 <= z0 + z2)%Z)


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