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


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