Goal:
    x : V.t  y : V.t  p : VSet.t  a : V.t  H : x = a \/ y = a \/ VSet.In a p
  ============================
   (y = a \/ x = a \/ VSet.In a p)


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