Goal:
    x : V.t  p : VSet.t  H : ~ VSet.In x p  a : V.t
  ============================
   (((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
           | H:e |- _ => fail 1
           | _ => assert e by t
           end