Goal:
  
  f : EdgeSet.elt -> VSet.t -> VSet.t
  prpf : Proper (eq ==> VSet.Equal ==> VSet.Equal) f
  ============================
   (forall (a : EdgeSet.elt) (l : list EdgeSet.elt),
    Proper (VSet.Equal ==> VSet.Equal)
      (fold_left (fun (a0 : VSet.t) (e : EdgeSet.elt) => f e a0) l) ->
    Proper (VSet.Equal ==> VSet.Equal)
      (fun a0 : VSet.t =>
       fold_left (fun (a1 : VSet.t) (e : EdgeSet.elt) => f e a1) l (f a a0)))


DEBUG:Going to execute:
<coq-core.plugins.ltac::trivial@0> $1 $2