Goal:
  
  ============================
   (nth_error ?l 0 = Some false)


DEBUG:Going to execute:
let __ := open_constr:((eq_refl : ev = _ :: _)) in
           idtac
Level 2: evaluation returns
<coq-core.plugins.ltac::is_evar@0>
where
$1 := ?l
of type
constr of type tacvalue