@mixin background-size ($length-1,
  $length-2: false, $length-3: false,
  $length-4: false, $length-5: false,
  $length-6: false, $length-7: false,
  $length-8: false, $length-9: false)
  {
    $full: compact($length-1, $length-2, $length-3, $length-4,
                   $length-5, $length-6, $length-7, $length-8, $length-9);

@include prefixer(background-size, $full, webkit, moz, ms, o);
}