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


DEBUG:Going to execute:
do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]