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')
  ============================
   (OnOne2i P (S (#|l| + i)) tl tl'
    → OnOne2i P i (a :: l ++ tl) (a :: l ++ tl'))


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

In environment
A : Type
P : nat → A → A → Type
i : nat
tl, tl' : list A
X : OnOne2i P i tl tl'
Unable to unify "OnOne2i ?M2652 ?M2653 (?M2654 :: ?M2655) (?M2654 :: ?M2656)"
with "OnOne2i P i tl tl'".
Level 0: Backtrace:

In environment
A : Type
P : nat → A → A → Type
i : nat
tl, tl' : list A
X : OnOne2i P i tl tl'
Unable to unify "OnOne2i ?M2652 ?M2653 (?M2654 :: ?M2655) (?M2654 :: ?M2656)"
with "OnOne2i P i tl tl'".