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
  ============================
   (forall (a : EdgeSet.elt) (l : list EdgeSet.elt),
    (forall i : VSet.t,
     VSet.Equal
       (fold_left (fun (a0 : VSet.t) (e : EdgeSet.elt) => f e a0) l (f x i))
       (f x (fold_left (fun (a0 : VSet.t) (e : EdgeSet.elt) => f e a0) l i))) ->
    forall i : VSet.t,
    VSet.Equal
      (fold_left (fun (a0 : VSet.t) (e : EdgeSet.elt) => f e a0) l
         (f a (f x i)))
      (f x
         (fold_left (fun (a0 : VSet.t) (e : EdgeSet.elt) => f e a0) l (f a i))))


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | Ltac.split ]
            | case not_locked_false_eq_true; Ltac.assumption
            | match goal with
              | H:~ _ |- _ => solve [ case H; trivial ]
              end ])