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


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