Goal:
    s' : VSet.t  t0 : VSet.t  a : VSet.elt  H : VSet.In a s'
  ============================
   (VSet.In a (VSet.union t0 s'))


DEBUG:Going to execute:
repeat
            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