Goal:
  
  A : Type
  B : Type
  f : nat → A → B
  l : list A
  a : A
  l' : list A
  IHl' : ∀ n : nat,
           mapi_rec f l' (#|l| + n) =
           mapi_rec (λ (n0 : nat) (x : A), f (#|l| + n0) x) l' n
  n : nat
  ============================
   (f (#|l| + n) a :: mapi_rec f l' (S #|l| + n) =
    mapi_rec (λ (n : nat) (x : A), f (#|l| + n) x) (a :: l') n)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2
DEBUG LTAC Evaluated term: mapi_rec_eqn
DEBUG LTAC Evaluated term: mapi_rec_eqn
DEBUG LTAC Evaluated term: (S (#|l| + n))
DEBUG LTAC Evaluated term: (S #|l| + n)