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
  ih : 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
  H : forall l : list EdgeSet.elt,
      Proper (VSet.Equal ==> VSet.Equal)
        (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l)
  ============================
   (VSet.Equal
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l
         (f x (f a i)))
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l
         (f x (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 ])