Goal:
    z : t  hd : A.t  tl : list A.t  H : lt (hd :: tl) z
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in lt [] z)


DEBUG:Going to execute:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')