Goal:
    A : Type  v : A  _a_ : A  _l_ : list A  p : Some _a_ = Some v
  ============================
   (_a_ = v ∨ ∃ n : nat, hd_error (skipn n _l_) = Some v)


DEBUG:Going to execute:
do
![ solve
 [ trivial | apply : eq_sym ; trivial ]
 | discriminate
 | contradiction
 | Ltac.split ]