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
  x : EdgeSet.elt
  l : list EdgeSet.elt
  ============================
   (VSet.Equal
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l (f x i))
      (f x (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) l i)))


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

No matching clauses for match.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.
Level 1: Backtrace:

No applicable tactic.
Level 0: Backtrace:

No applicable tactic.