Taxonomy of Categories
	
	Facets are defined using a hierarchy of categories, known as a Taxonomy.
	For example, the taxonomy of a book store application might have the following structure:
	
	
	
	The Taxonomy translates category-paths into interger identifiers (often termed ordinals) and vice versa.
	The category Author/Mark Twain adds two nodes to the taxonomy: Author and 
	Author/Mark Twain, each is assigned a different ordinal. The taxonomy maintains the invariant that a 
	node always has an ordinal that is < all its children.