Goal:
  
  z : Z
  z0 : Z
  k : t
  H : Some (Z.max z z0) = k
  H1 : (z0 <= z)%Z
  H2 : Z.max z z0 = z
  ============================
   (Some z = k \/ Some z0 = k)


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