Goal:
  
  A : Type
  f : nat → A
  m : nat
  n : nat
  t : A
  IHm : ∀ n : nat, nth_error (unfold m f) n = Some t → t = f n
  Hn : nth_error (unfold m f ++ [f m]) n = Some t
  H : n < #|unfold m f ++ [f m]|
  ============================
   (t = f n)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
DEBUG LTAC Evaluated term: (nth_error_Some_length Hn)

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]
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]