Goal:
    A : Type  n : nat  hd : A  tl : list A  H : S n < #|hd :: tl|
  ============================
   (S n ≤ #|tl|)


DEBUG:Going to execute:
fun H =>
             let ph := fresh "X" in
             let ph' := fresh "X" in
             destruct H as [H ph ph']