Goal:
    A : Type  f : nat → A  n : nat  t : A  Hn : None = Some t
  ============================
   (t = f n)


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
DEBUG LTAC Evaluated term: nth_error_nil
DEBUG LTAC Evaluated term: nth_error_nil

File "./theories/MCList.v", line 1065, characters 12-22:
Warning: Notation app_length is deprecated since 8.20.
Use length_app instead.
[deprecated-syntactic-definition-since-8.20,deprecated-since-8.20,deprecated-syntactic-definition,deprecated,default]