Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  i : list B
  tl : list A
  tl' : list A
  ============================
   (OnOne2All P i tl tl' → #|[]| = 0 → OnOne2All P ([] ++ i) tl tl')


DEBUG:Going to execute:
trivial