defmethod+.clj

(defmulti repeating-mm (juxt first second))
(defmethod repeating-mm [1 2] [_] (str [1 2]))

(repeating-mm (range 1 10))
;=> "[1 2]"

(defmacro defmethod-anaphoric
  [multifn dispatch-val & fn-tail]
  `(. ~(with-meta multifn {:tag 'clojure.lang.MultiFn}) 
      addMethod 
      ~dispatch-val
      (let [~'$ ~dispatch-val]
        (fn ~@fn-tail))))

(defmulti a-mm (juxt first second))
(defmethod-anaphoric a-mm [1 2] [_] (str $))
(defmethod-anaphoric a-mm [3 4] [s] (reverse s))

(a-mm [1 2])
;=> "[1 2]"

(a-mm [3 4])
;=> (4 3)


(defmacro defmethod-explicit
  [multifn dispatch-val & fn-tail]
  (let [[kw n & body] fn-tail]
    (if (= :as kw)
      `(. ~(with-meta multifn {:tag 'clojure.lang.MultiFn}) 
          addMethod 
          ~dispatch-val 
          (let [~n ~dispatch-val] (fn ~@body)))
      `(. ~(with-meta multifn {:tag 'clojure.lang.MultiFn}) 
          addMethod 
          ~dispatch-val 
          (fn ~@fn-tail)))))

(defmulti expl-mm (juxt first second))
(defmethod-explicit expl-mm [1 2] :as dv [_] (str dv))
(defmethod-explicit expl-mm [3 4] [s] (reverse s))

(expl-mm [1 2])
;=> "[1 2]"

(expl-mm [3 4])
;=> (4 3)