Goal:
  
  A : Type
  v : A
  _a_ : A
  _l_ : list A
  _n_ : nat
  p : hd_error (skipn _n_ _l_) = 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