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


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