Goal:
    A : Type  P : A → Type  l : list A  n : nat
  ============================
   (All P l → All P (firstn n l))


DEBUG:Going to execute:
intros HPL; induction HPL in n |- *; simpl; destruct n;
            try econstructor
DEBUG LTAC Evaluated term: All_Forall
DEBUG LTAC Evaluated term: (All_Forall (λ x : A, p x) l _view_subject_)
DEBUG LTAC Evaluated term: forallb_Forall