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


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

File "./theories/MCList.v", line 1056, 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 1056, 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]