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


DEBUG:Going to execute:
match goal with
           | H:?T
             |- _ =>
                 progress change T with V.t in H;
                  repeat
                   match goal with
                   | J:_ |- _ => progress change T with V.t in J
                   | |- _ => progress change T with V.t
                   end
           | H:forall x : ?T, _
             |- _ =>
                 progress change T with V.t in H;
                  repeat
                   match goal with
                   | J:_ |- _ => progress change T with V.t in J
                   | |- _ => progress change T with V.t
                   end
           end