Goal:
  
  A : Type
  P : nat → A → A → Type
  i : nat
  hd : A
  hd' : A
  tl : list A
  p : P i hd hd'
  X : A → A → Type
  ============================
   (hd' :: tl = [] ++ hd' :: tl)


DEBUG:Going to execute:
clear id0