Clojurians Log v2

Regarding fact-type customization, from here: https://github.com/cerner/clara-examples/blob/master/src/main/clojure/clara/examples/fact_type_options.clj

Can someone explain how this works to find ancestors?

(def custom-ancestors-fn {:precise-temperature-reading #{:temperature-reading}})

I get that a map can work in the function position, and will look up the value for a key given as argument, but I am struggling to see how this def works as a function to look up ancestors.