Goal:
  
  A : Type
  v : A
  a : A
  l : list A
  H : (∃ n : nat, hd_error (skipn n l) = Some v) ↔ In v l
  ============================
   ((∃ n : nat, hd_error (skipn n (a :: l)) = Some v) ↔ a = v ∨ In v l)


DEBUG:Going to execute:
solve
[ do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]
| case not_locked_false_eq_true; Ltac.assumption
| match goal with
  | H:¬ _ |- _ => solve [ case H; trivial ]
  end ]