Infinitely deep facet/term combinations

We sometimes wonder whether or not the facet/term combinations should be infinitely deep.

As of right now the facet/term combinations are simple two-dimensional hierarchies. Examples include:

Audience/Freshman
Audience/Sophomore
Audience/Junior

Tools/Dictionaries
Tools/Catalogs
Tools/Indexes

Formats/Books
Formats/Journals
Formats/Movies

These two-dimensional hierarchies work, most of the time, but what do you do when it comes to subjects? This works just fine, as far as it goes:

Subjects/Astronomy
Subjects/Mathematics
Subjects/Literature
Subjects/Music

To make a subjects hierarchy […]