Goal:
  
  A : Type
  B : Type
  f : nat → B
  a : A
  l : list A
  IHl : ∀ f : nat → B,
          mapi_rec (λ (i : nat) (_ : A), f i) l 0 = unfold #|l| f
  ============================
   (S #|l| = #|l ++ [a]|)


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

File "./theories/MCList.v", line 1038, characters 40-50:
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 1038, characters 40-50:
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]