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
  i : VSet.t
  ============================
   (VSet.Equal (f x i) (f x i))


DEBUG:Going to execute:
apply : eq_sym ; trivial
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.