Goal:
  
  A : Type
  sizeA : A → nat
  B : Type
  sizeB : B → nat
  l : list A
  f : nat → A → B
  H : ∀ (i : nat) (x : A), sizeB (f i x) = sizeA x
  ============================
   (list_size sizeB (mapi_rec f l 0) = list_size sizeA l)


DEBUG:Going to execute:
t; easy