Forever ago I built something similar for generating canonical urls for SEO from a tag system (and to concentrate "google juice" from linking), though hierarchi

Forever ago I built something similar for generating canonical urls for SEO from a tag system (and to concentrate "google juice" from linking), though hierarchies were not strictly enforced. "expert/american" and "american/expert" couldn't have duplicate content so you take your tag system and overlay hierarchies. Anything that wasn't manually set was auto hierarchied based on highest traffic volume. I also prevented tags from showing up on the same tag chain, so any given keyword could only appear once. That prevented infinite recursion. Moving away from hierarchy also made interesting permutations easier to generate. Since any metadata can become a tag, price or price range ($100hr – $300hr) is easy to generate and "enhance" American/Experts/Between$100and$300anhour. It worked really well, allowed us to manually enforce high traffic hierarchies/phrases while still auto-generating intelligent canonical links for the rest of the site.