Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  z : V.t
  x : V.t
  e : EdgeOf G x z
  i : VSet.elt
  H : VSet.In i (VSet.add x VSet.empty)
  ============================
   (VSet.In i VSet.empty \/ i = x)


DEBUG:Going to execute:
idtac
Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.