Goal:
    A : Type  l : list A  n : nat  t : A
  ============================
   (nth_error l n = Some t → nth_error l n ≠ None)


DEBUG:Going to execute:
destruct (nth_error l n)
DEBUG LTAC Evaluated term: nth_error_Some
DEBUG LTAC Evaluated term: nth_error_Some

Goals:
    A : Type  l : list A  n : nat  t : A  a : A
  ============================
   (Some a = Some t → Some a ≠ None)

    A : Type  l : list A  n : nat  t : A
  ============================
   (None = Some t → None ≠ None)


DEBUG:Going to execute:
congruence
DEBUG LTAC Evaluated term: (nth_error l n)

Goals:
    A : Type  l : list A  n : nat  t : A  a : A
  ============================
   (Some a = Some t → Some a ≠ None)

    A : Type  l : list A  n : nat  t : A
  ============================
   (None = Some t → None ≠ None)


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