Goal:
  
  A : Type
  P : A → Prop
  x : A
  l : list A
  H : P x
  H0 : Forall P l
  IHForall : ∀ n : nat, Forall P (skipn n l)
  ============================
   (Forall P (x :: l))


DEBUG:Going to execute:
constructor
DEBUG LTAC Evaluated term: skipn
DEBUG LTAC Evaluated term: skipn