Goal:
    G : t  HI : invariants  x : VSet.elt  Hx : VSet.In x (V G)
  ============================
   (true = true)


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"