Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  n : nat
  e : nth_error [] (S n) = None
  IHn : ∀ (l : list A) (l' : list B),
          nth_error l n = None → Forall2 P l l' → nth_error l' n = None
  ============================
   (nth_error [] (S n) = None)


DEBUG:Going to execute:
fun => <coq-core.plugins.ltac::reflexivity@0>