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


DEBUG:Going to execute:
<coq-core.plugins.ltac::trivial@0> $1 $2