要编写一个多元的defn宏，可以使用Clojure的宏定义功能来实现。下面是一个示例：

(defmacro defn-multi
  [name & clauses]
  (let [arity->body (into {} (mapcat parse-clause clauses))]
    `(defn ~name
       ([~'& args#]
        (let [n (count args#)]
          (if-let [body# (get ~arity->body n)]
            (apply body# args#)
            (throw (IllegalArgumentException.
                    (str "Invalid arity: " n)))))))
  )

(defn parse-clause
  [clause]
  (let [[args body] (split-with #(not= % '&) clause)]
    (if (empty? body)
      (throw (IllegalArgumentException.
              "No function body provided"))
      {(count args) `(fn ~args ~@body)}))
  )

使用这个defn-multi宏，可以定义多个参数个数的函数。例如：

(defn-multi foo
  ([] "No arguments")
  ([x] (str "One argument: " x))
  ([x y] (str "Two arguments: " x " and " y))
  ([x y z] (str "Three arguments: " x ", " y " and " z)))

(foo)          ; "No arguments"
(foo 1)        ; "One argument: 1"
(foo 1 2)      ; "Two arguments: 1 and 2"
(foo 1 2 3)    ; "Three arguments: 1, 2 and 3"
(foo 1 2 3 4)  ; throws IllegalArgumentException

这个示例中，defn-multi宏接受一个函数名和多个参数个数与函数体的对应的参数，然后生成一个defn形式的函数定义。在运行时，通过检查参数的个数，根据参数个数调用对应的函数体。如果给定的参数个数没有匹配的函数体，则抛出一个IllegalArgumentException异常。