Goal:
  
  A : Type
  B : Type
  p : B → bool
  f : nat → B
  l : list A
  Hp : ∀ i : nat, i < #|l| → p (f i)
  ============================
   (is_true (forallb p (unfold #|l| f)))


DEBUG:Going to execute:
induction #|l| in *; simpl
DEBUG LTAC Evaluated term: (mapi_unfold f)
DEBUG LTAC Evaluated term: (mapi_unfold f)