def-jambda with signatures next to the body

def-jambda-ex.rkt

(defn
  #:doc @list{Multiplies @racket[x] by @racket[y].
              Use it fruitfully.}
  #:ex [10 3 => 30]
  #:ex [0 3 => 0]
  #:ex [10 => 10]
  #:ex [0 => 0]
  ;; The function signature
  (mult [x number?][y number? 1] -> number?)
    ;; And the function body:
    (* x 3))