Goal:
  
  G : t
  s0 : VSet.t
  x : VSet.elt
  Hx : VSet.In x s0
  s1 : VSet.t
  s2 : VSet.t
  Hs : VSet.Equal (VSet.union s0 (VSet.add x s1)) s2
  y : VSet.elt
  Hy : VSet.In y s1
  ============================
   (VSet.In y s1)


DEBUG:Going to execute:
let rec use_hyps :=
            match goal with
            | H:_ /\ _ |- _ => exact H || (destruct_hyp H; use_hyps)
            | H:(_ * _)%nat |- _ => exact H || (destruct_hyp H; use_hyps)
            | H:_ |- _ => solve [ inversion H ]
            | _ => idtac
            end
           in
           let do_atom := solve
           [ trivial
           with eq_true
           | reflexivity
           | symmetry; trivial
           | contradiction
           | congruence ]
           in
           let rec do_ccl :=
            try do_atom; repeat (do_intro; try do_atom); try arith_congr;
             (solve [ split; do_ccl ])
           in
           (solve [ do_atom | use_hyps; do_ccl ]) ||
             fail "Cannot solve this goal"