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


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrtclby@0> $1
Level 0: Backtrace:

No matching clauses for match.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 1: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
DEBUG LTAC Evaluated term: (forall l : list EdgeSet.elt,
                            Proper (VSet.Equal ==> VSet.Equal)
                              (fold_left
                                 (fun (a : VSet.t) (e : EdgeSet.elt) => f e a)
                                 l))