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:
use_hyps; do_ccl
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

No applicable tactic.