Goal:
    x : V.t  p : VSet.t  H : VSet.In x p -> False  a : V.t  H0 : x = a
  ============================
   ((x = a \/ VSet.In a p) /\ x <> a -> VSet.In a p)


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