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


DEBUG:Going to execute:
<coq-core.plugins.ltac::intro@1> $1