Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  i : list B
  b : B
  i' : list B
  tl : list A
  tl' : list A
  X : OnOne2All P i tl tl'
  H : #|b :: i'| = 0
  ============================
   (OnOne2All P ((b :: i') ++ i) tl tl')


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

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.