Goal:
  
  A : Type
  sizeA : A → nat
  B : Type
  sizeB : B → nat
  a : A
  l : list A
  f : nat → A → B
  k : nat
  H : sizeB (f k a) ≤ sizeA a
  IHl : list_size sizeB (mapi_rec f l (S 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:
idtac