SASS generate nav spritesheet

/* ********* nav *********** */
/* required for nav_mixin */
/*	$nav_height: 50px */
/*	$nav_button_width: 133px */
/*	$button_count: 6 */

@mixin sprite($i)
	width: $nav_button_width
	background-position: -1 * ($i - 1) * $nav_button_width 0
	&:hover
		background-position: -1 * ($i - 1) * $nav_button_width -1 * $nav_height
	&:active
		background-position: -1 * ($i - 1) * $nav_button_width -2 * $nav_height


#nav
	width: $content_width
	height: $nav_height
	margin: 0 auto
	#menu_items
		height: $nav_height
		margin: 0 auto
		cursor: pointer
		.nav_item
			width: $nav_button_width
			height: $nav_height
			float: left
			position: relative
			background: url('../images/nav_sprites.jpg') no-repeat scroll 0 0
			border: none
		@for $i from 1 through $button_count
			#nav1
				@include sprite($i)