(prefers multifn)
Given a multimethod, returns a map of preferred value -> set of other values
(def m {:os ::osx})
(defmulti ex :os)
(defmethod ex ::unix
[_]
"unix")
(derive ::osx ::unix)
(defmethod ex ::bsd
[_]
"bsd")
(derive ::osx ::bsd)
(prefer-method ex ::unix ::bsd)
(prefers ex)
;;=> {:user/unix #{:user/bsd}}