Goal:
  
  G1 : t
  ext : EdgeSet.t
  f : EdgeSet.elt -> VSet.t -> VSet.t
  trf : transpose VSet.Equal f
  prpf : Proper (eq ==> VSet.Equal ==> VSet.Equal) f
  x : EdgeSet.elt
  a : EdgeSet.elt
  l : list EdgeSet.elt
  H : forall i : VSet.t,
      VSet.Equal
        (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l (f x i))
        (f x (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l i))
  i : VSet.t
  ============================
   (VSet.Equal
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l
         (f a (f x i)))
      (f x
         (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l (f a i))))


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.