Mixin for responsive sprites with Sass

gistfile1.scss

@mixin responsive-sprite($map, $icon){
  $icon-file: sprite-file($map, $icon);
  $icon-width: image-width($icon-file);
  $icon-height: image-height($icon-file);

  $sprite-file: sprite-path($map);
  $sprite-width: image-width($sprite-file);
  $sprite-height: image-height($sprite-file);

  $space-top: floor(nth(sprite-position($map, $icon), 2));
  @if $space-top == 0 {
    $space-top: 0px
  }

  width:percentage($sprite-width / $icon-width);
  display: block;
  height: 0;
  padding-bottom: percentage($icon-height / $icon-width);
  background-size: 100%;
  background-position:0 percentage(
    -1 * $space-top / ( $sprite-height - $icon-height )
  );
}

Sorry, I am new in sprites images.
When I compile I get Undefined variable: "$icon-sprites"
Could you tell me how to define this variable?
I am using the same example as you.

Finally, I got it!
But I want the same request as @nicolasiensen
This would be great the image respect the original width and it doesn't span the whole of its container