based on hello from Stuart Halloway's book

gistfile1.clj

;; reference to a set (unique) visitors
(def visitors (ref #{}))

(defn hello 
  "Writes hello message to *out*. Calls you by username.
  Knows if you have been here before."
  ([] (hello "world"))   ; with no arguments
  ([username]
  (dosync
    ;; deref visitors, call username to see if it's there
    (let [past-visitor (@visitors username)]
      (if past-visitor
        (str "Welcome back, " username)
      ;; else
        (do
          ;; conjoin (append) an element
          (alter visitors conj username)
          (str "Hello, " username)))))))

(hello "Nathan")
(hello "Nathan")
(hello)