Goal:
  
  A : Type
  a : A
  x : A
  l : list A
  IHl : 0 < S #|l| → Some a = nth_error (List.rev l ++ [a]) (#|l| - 0)
  ============================
   (0 < S #|l ++ [x]|
    → Some a = nth_error (List.rev (l ++ [x]) ++ [a]) (#|l ++ [x]| - 0))


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

File "./theories/MCList.v", line 859, characters 10-20:
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]