(def include? (thing things)
  ; alias for 'detect
  ; true if thing is in things, nil otherwise
  (detect thing things))

(def sort-by (f things)
  ; sort 'things according to the value
  ; returned by 'f for each thing in 'things
  (let tmp (hash)
    (each thing things
          (hash-cons tmp
                    (f thing)
                    thing))
    (apply joinlists
           (map λx(hash-get tmp x)
                (sort:hash-keys tmp)))))