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:
reflexivity
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.