Goal:
  
  A : Type
  P : A → A → Type
  a : A
  l : list A
  tl : list A
  tl' : list A
  IHl : OnOne2 P tl tl' → OnOne2 P (l ++ tl) (l ++ tl')
  ============================
   (OnOne2 P tl tl' → OnOne2 P (a :: l ++ tl) (a :: l ++ tl'))


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

In environment
A : Type
P : A → A → Type
tl, tl' : list A
X : OnOne2 P tl tl'
Unable to unify "OnOne2 ?M2567 (?M2568 :: ?M2569) (?M2568 :: ?M2570)" with
 "OnOne2 P tl tl'".
Level 0: Backtrace:

In environment
A : Type
P : A → A → Type
tl, tl' : list A
X : OnOne2 P tl tl'
Unable to unify "OnOne2 ?M2567 (?M2568 :: ?M2569) (?M2568 :: ?M2570)" with
 "OnOne2 P tl tl'".