gistfile1.txt

mr1.mr1=> (union #{1 2 3} #{4 5 6})
CompilerException java.lang.RuntimeException: Unable to resolve symbol: union in this context, compiling:(NO_SOURCE_PATH:3) 
mr1.mr1=> (clojure.set/union #{1 2 3} #{4 5 6})
ClassNotFoundException clojure.set  java.net.URLClassLoader$1.run (URLClassLoader.java:202)
mr1.mr1=> (require 'clojure.set)
nil
mr1.mr1=> (clojure.set/union #{1 2 3} #{4 5 6})
#{1 2 3 4 5 6}
mr1.mr1=> (require '[clojure.set :as set])
nil
mr1.mr1=> (set/union #{1 2 3} #{4 5 6})        
#{1 2 3 4 5 6}