Goal:
  
  A : Type
  P : nat → A → Type
  f : A → A
  n : nat
  H : ∀ (n : nat) (x : A), P n x → f x = x
  ============================
   ([] = [])


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