Goal:
  
  G1 : t
  ext : EdgeSet.t
  i : VSet.t
  f : EdgeSet.elt -> VSet.t -> VSet.t
  trf : transpose VSet.Equal f
  prpf : Proper (eq ==> VSet.Equal ==> VSet.Equal) f
  a : EdgeSet.elt
  l : list EdgeSet.elt
  H : VSet.Equal
        (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l i)
        (fold_right f i l)
  a0 : VSet.elt
  ============================
   (VSet.In a0 (fold_right f i (a :: l)) ->
    VSet.In a0
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) (a :: l) i))


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

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.