Goal:
  
  x : V.t
  p : VSet.t
  H : VSet.In x p -> False
  a : V.t
  H0 : x = a
  H1 : 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