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:
Tactics.destruct_one_pair || Tactics.destruct_one_ex
Level 7: evaluation returns
destruct H as [H ph]
where
ph := H
of type
intropattern
H := n0
of type
constr of type tacvalue