reload a multimethod in REPL

(defmulti greeting :language)
(defmethod greeting "English" [_] "Hello!")

(greeting {:language "English"})
;=> Hello!


;; At this point, we realized that we want to use `:lang`, not `:language`...
(defmulti greeting :lang)
(defmethod greeting "English" [_] "Hello!")

(greeting {:lang "English"})
;=> IllegalArgumentException No method in multimethod 'greeting' for dispatch value: null  clojure.lang.MultiFn.getFn (MultiFn.java:156)


;;  to resolve
(def greeting nil)
; or use ns-unmap
(ns-unmap *ns* 'greeting)

; then re-define
(defmulti greeting :lang)
(defmethod greeting "English" [_] "Hello!")

(greeting {:lang "English"})
;=> Hello!