Goal:
  
  A : Type
  l : list A
  n0 : nat
  H : n0 < #|l|
  hd : A
  tl : list A
  Heq_l : hd :: tl = l
  n : nat
  Heq_n : S n = proj1_sig (exist (λ n : nat, n < #|l|) n0 H)
  ============================
   (n < #|tl|)


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