Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  l : list A
  l' : list B
  n : nat
  Hall : All2 P l l'
  ============================
   (nth_error l n = None → nth_error l' n = None)


DEBUG:Going to execute:
<coq-core.plugins.ltac::revert@0> $1