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)
  ============================
   (VSet.Equal
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) (a :: l) i)
      (fold_right f i (a :: l)))


DEBUG:Going to execute:
trivial