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:
let tac2 H :=
            let ph := fresh "H" in
            let ph' := fresh "H" in
            destruct H as [H ph ph']
           in
           let tacT H := let ph := fresh "X" in
                         destruct H as [H ph] in
           let tacT2 H :=
            let ph := fresh "X" in
            let ph' := fresh "X" in
            destruct H as [H ph ph']
           in
           match goal with
           | H:∃ y, _ y |- _ => tac H
           | H:{x : _ | ?P x} |- _ => tac H
           | H:∑ y, ?P y |- _ => tacT H
           | H:exists2 x : _, _ x & _ x |- _ => tac2 H
           | H:{x : _ | ?P x & _ x} |- _ => tac2 H
           | H:{x : _ & ?P x & _ x} |- _ => tacT2 H
           end