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


DEBUG:Going to execute:
trivial; hnf; intros