Goal:
  
  A : Type
  B : Type
  p : B → bool
  f : nat → B
  l : list A
  n : nat
  Hp : ∀ i : nat, i < S n → p (f i)
  IHn : (∀ i : nat, i < n → p (f i)) → forallb p (unfold n f)
  ============================
   (is_true (forallb p (unfold n f ++ [f n])))


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2