Goal:
  
  A : Type
  P : nat → A → A → Type
  i : nat
  l : list A
  tl : list A
  tl' : list A
  ============================
   (OnOne2i P (#|l| + i) tl tl' → OnOne2i P i (l ++ tl) (l ++ tl'))


DEBUG:Going to execute:
induction l in i |- *; simpl; try constructor