Goal:
    x : V.t  p : VSet.t  H : ~ VSet.In x p  a : V.t  H1 : x = a \/ x <> a
  ============================
   (((x = a \/ VSet.In a p) /\ x <> a -> VSet.In a p) /\
    (VSet.In a p -> (x = a \/ VSet.In a p) /\ x <> a))


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