q.clj

(ns whee
  (:refer-clojure :exclude [conj]))

(defprotocol Q
  (conj [q elem]))

(extend-type clojure.lang.PersistentQueue
  Q
  (conj [this elem]
    (if (>= (count this) 3)
      this
      (clojure.core/conj this elem))))

(-> clojure.lang.PersistentQueue/EMPTY
    (conj :a)
    (conj :b)
    (conj :c)
    pop
    (conj :d)
    (conj :z)
    seq)

;=> (:b :c :d)


(ns yay
  (:refer whee))

(-> clojure.lang.PersistentQueue/EMPTY
    (conj :a)
    (conj :b)
    (conj :c)
    pop
    (conj :d)
    (conj :z)
    seq)
;=> (:b :c :d)

(-> clojure.lang.PersistentQueue/EMPTY
    (clojure.core/conj :a)
    (clojure.core/conj :b)
    (clojure.core/conj :c)
    pop
    (clojure.core/conj :d)
    (clojure.core/conj :z)
    seq)

;=> (:b :c :d :z)