Goal:
  
  y : V.t
  s0 : VSet.t
  s1 : VSet.t
  eq : VSet.Equal s0 s1
  s0' : VSet.t
  s1' : VSet.t
  eq' : VSet.Equal s0' s1'
  ============================
   (VSetProp.Add y s1 s1' /\ ~ VSet.In y s1 <->
    VSetProp.Add y s1 s1' /\ ~ VSet.In y s1)


DEBUG:Going to execute:
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"