The top level index is a clojure map (a persistent hash-map by default i believe) of strings (stems) to another map of key to weighting. The hash-map is impleme

The top level index is a clojure map (a persistent hash-map by default i believe) of strings (stems) to another map of key to weighting. The hash-map is implemented as a Bagwell Trie under the hood. Check out http://blog.higher-order.net/2009/09/08/understanding-clojures-persistenthashmap-deftwice/ http://blog.higher-order.net/2009/09/08/understanding-clojur... for a lot of detail on the implementation, and http://en.wikipedia.org/wiki/Trie http://en.wikipedia.org/wiki/Trie for a general overview of how Tries differ from Trees.