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:
<coq-core.plugins.ltac::discriminate@0>
Level 0: Backtrace:

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym