Goal:
  
  s : VSet.t
  x : VSet.elt
  H : VSet.In x s
  z : VSet.elt
  Hz : VSet.In z s
  n : x <> z
  ============================
   (VSet.In z s)


DEBUG:Going to execute:
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"