Goal:
  
  A : Type
  n : nat
  t' : A
  P : A → A → Type
  hd : A
  hd' : A
  tl : list A
  p : P hd hd'
  H : nth_error tl n = Some t'
  ============================
   (nth_error tl n = Some t' × (t' = t') + P t' t')


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