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


DEBUG:Going to execute:
induction l; simpl