Group Products by region when you don't know the set of regions

<?xml version="1.0" encoding="utf-8"?>
<products>
	<product>
		<prodid>V667320</prodid>  <name>Battery Pack</name>
		<price>$20</price>        <quantity>178</quantity>
		<region>SouthWest</region>
	</product>
	<product>
		<prodid>M382932</prodid>  <name>CD Visor Organizer</name>
		<price>$19</price>  <quantity>129</quantity>
		<region>NorthWest</region>
	</product>
	<product>
		<prodid>U3923839</prodid>  <name>Rock n Roll Supercart</name>
		<price>$150</price>   <quantity>200</quantity>
		<region>SouthWest</region>
	</product>
	<product>
		<prodid>Q3929302</prodid>   <name>Digital Tire Gauge</name>
		<price>$40</price>   <quantity>623</quantity>
		<region>NorthWest</region>
	</product>
.......
</products>

...but we don't know how many regions there are!