Goal:
  
  A : Type
  sizeA : A → nat
  B : Type
  sizeB : B → nat
  a : A
  l : list A
  f : nat → A → B
  k : nat
  H : ∀ (i : nat) (x : A), sizeB (f i x) = sizeA x
  IHl : ∀ k : nat, list_size sizeB (mapi_rec f l k) = list_size sizeA l
  ============================
   (S (sizeB (f k a) + list_size sizeB (mapi_rec f l (S k))) =
    S (sizeA a + list_size sizeA l))


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