Goal:
  
  A : Type
  p : A → bool
  n : nat
  x : A
  HPl : is_true (forallb p [])
  Hnth : None = Some x
  ============================
   (is_true (p x))


DEBUG:Going to execute:
done
DEBUG LTAC Evaluated term: nth_error_nil
DEBUG LTAC Evaluated term: nth_error_nil