Goal:
  
  G1 : t
  ext : EdgeSet.t
  i : VSet.t
  f : EdgeSet.elt -> VSet.t -> VSet.t
  trf : transpose VSet.Equal f
  prpf : Proper (eq ==> VSet.Equal ==> VSet.Equal) f
  a : VSet.elt
  ============================
   (VSet.In a (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) [] i) <->
    VSet.In a (fold_right f i []))


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

No primitive equality found.