Goal:
  
  A : Type
  f : A → bool
  g : nat → A
  n : nat
  fg : ∀ x : nat, x < S n → f (g x)
  IHn : (∀ x : nat, x < n → f (g x)) → forallb f (unfold n g)
  x : nat
  lt : x < n
  ============================
   (is_true (f (g x)))


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