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


DEBUG:Going to execute:
<Coq.Init.Tauto.axioms> flags
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

Tactic failure: congruence failed.