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# 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:

*   Author

    *   Mark Twain

    *   J. K. Rowling

*   Date

    *   2010

    *   March

    *   April

    *   2009

	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.