Grouping a List into A Grid Structure

<root>
	<list>
		<item type="a">list 1 item 1</item>
		<item type="b">list 1 item 2</item>
	</list>
	<list>
		<item type="a">list 2 item 1</item>
		<item type="b">list 2 item 2</item>
		<item type="b">list 2 item 3</item>
		<item type="b">list 2 item 4</item>
	</list>
	<list>
		<item type="a">list 3 item 1</item>
		<item type="b">list 3 item 2</item>
		<item type="b">list 3 item 3</item>
	</list>
</root>

<root>
	<list>
		<item type="a">list 1 item 1</item>
		<item type="a">list 2 item 1</item>
		<item type="a">list 3 item 1</item>
		<item type="b">list 1 item 2</item>
		<item/>
		<item/>
		<item/>
		<item type="b">list 2 item 2</item>
		<item type="b">list 3 item 2</item>
		<item/>
		<item type="b">list 2 item 3</item>
		<item type="b">list 3 item 3</item>
		<item/>
		<item type="b">list 2 item 4</item>
		<item/>
	</list>
</root>

<xsl:template match="root">
	<xsl:for-each-group select="list/item" group-by="@type">
		<xsl:choose>
			<xsl:when test="current-grouping-key()='a'">
				<xsl:copy-of select="current-group()"/>
			</xsl:when>
			<xsl:otherwise>
				<!-- grouping logic here -->
			</xsl:otherwise>
		</xsl:choose>
	</xsl:for-each-group>
</xsl:template>

Thanks in advance,
-- Jeff