Goal:
  
  A : Type
  P : nat → A → A → Type
  i : nat
  a : A
  l : list A
  tl : list A
  tl' : list A
  IHl : ∀ i : nat,
          OnOne2i P (#|l| + i) tl tl' → OnOne2i P i (l ++ tl) (l ++ tl')
  X : OnOne2i P (S (#|l| + i)) tl tl'
  ============================
   (OnOne2i P (S (#|l| + i)) tl tl')


DEBUG:Going to execute:
easy0 || (solve [ intuition eauto  3 with core terms ])