Goal:
  
  A : Type
  B : Type
  P : B → B → Prop
  f : A → B
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 (on_rel P f) tl tl'
  IHX : OnOne2 P (map f tl) (map f tl')
  ============================
   (OnOne2 P (map f tl) (map f tl'))


DEBUG:Going to execute:
congruence
Level 0: Backtrace:

Tactic failure: congruence failed.
Level 0: Backtrace:

Tactic failure: congruence failed.