I'm new to Clojure and I'm doing some basic stuff from labrepl, now I want to write a function that will replace certain letters with other letters, for example: elosska → elößkä.

I wrote this:

```
(ns student.dialect (:require [clojure.string :as str]))
(defn germanize
  [sentence]
  (def german-letters {"a" "ä" "u" "ü" "o" "ö" "ss" "ß"})
  (doseq [[original-letter new-letter] german-letters]
    (str/replace sentence original-letter new-letter)))

```

but it doesn't work as I expect. Could you help me, please?