Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  b : bool
  H0 : is_true b
  H : is_true (~~ true)
  ============================
   False


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"