Goal:
  
  x : V.t
  s : VSet.t
  s' : VSet.t
  z : VSet.elt
  H : VSetProp.Add x s (VSet.remove z s')
  H0 : VSet.In x s -> False
  i : VSet.elt
  ============================
   (VSet.In i s <-> VSet.In i s /\ ~ VSet.E.eq i z)


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