Goal:
  
  f : EdgeSet.elt -> VSet.t -> VSet.t
  prpf : Proper (eq ==> VSet.Equal ==> VSet.Equal) f
  _a_ : EdgeSet.elt
  _l_ : list EdgeSet.elt
  ih : Proper (VSet.Equal ==> VSet.Equal)
         (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) _l_)
  _x_ : VSet.t
  _y_ : VSet.t
  ============================
   (VSet.Equal
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) _l_
         (f _a_ _y_))
      (fold_left (fun (a : VSet.t) (e : EdgeSet.elt) => f e a) _l_
         (f _a_ _y_)))


DEBUG:Going to execute:
done
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.