Hoverable in img sprite (interesting mixin)

gistfile1.sass

@mixin hoverable($width, $height)
  display: block
  position: relative
  overflow: hidden
  height: $height
  width: $width
  img
    position: relative
    display: block
  img.rollover
    bottom: $height * 3
    opacity: 0
    filter: alpha(opacity=0)