Goal:
  
  A : Type
  B : Type
  f : nat -> A -> option B
  l : list A
  l' : list B
  i : nat
  t : B
  ============================
   (forall n : nat,
    map_option_out (mapi_rec f l n) = Some l' ->
    nth_error l' i = Some t ->
    ∑ x : A, nth_error l i = Some x /\ f (i + n)%nat x = Some t)


DEBUG:Going to execute:
induction l in i, l' |- *; intros n
DEBUG LTAC Evaluated term: (Nat.add_0_r i)
DEBUG LTAC Evaluated term: (Nat.add_0_r i)
DEBUG LTAC Evaluated term: 0