Goal:
    x : V.t  s : VSet.t  y : V.t
  ============================
   ((x = y \/ VSet.In y s -> x = y \/ VSet.In y s /\ x <> y) /\
    (x = y \/ VSet.In y s /\ x <> y -> x = y \/ VSet.In y s))


DEBUG:Going to execute:
match goal with
           | H:e |- _ => fail 1
           | _ => assert e by t
           end