union and difference of sets in Clojure -

i reading clojure section of book seven languages in 7 weeks. says:

you can merge 2 sets, this: user=> (clojure.set/union #{:skywalker} #{:vader}) #{:skywalker :vader} or compute difference: (clojure.set/difference #{1 2 3} #{2}) 

this doesn't work on version (clojure 1.7.0 , java version"1.8.0_51" ):

user=> (clojure.set/difference #{1 2 3} #{4}) classnotfoundexception clojure.set  java.net.urlclassloader.findclass (:-1)  user=> (clojure.set/union #{:skywalker} #{:vader}) classnotfoundexception clojure.set  java.net.urlclassloader.findclass (:-1) 

in clojuredocs shorter function names used in examples, doesn't work:

user=> (difference #{1 2} #{2 3}) compilerexception java.lang.runtimeexception: unable resolve symbol: difference in context, compiling:(no_source_path:14:1)  

why receiving these errors , how should fix that?

namespaces other clojure.core , user loaded if loads them. standard way load namespace require.

(require '[clojure.set :as set])  (set/union ...)