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


DEBUG:Going to execute:
assert X2; [ exact (id0 id1) | clear id0 ]