Goal:
  
  A : Type
  p : A → bool
  n : nat
  a : A
  l : list A
  IHl : ∀ n : nat, forallb p l → forallb p (skipn n l)
  H : p a ∧ forallb p l
  ============================
   (is_true (forallb p l))


DEBUG:Going to execute:
<Coq.Init.Tauto.is_conj> flags X1; elim id; do 2 intro; clear id