user=> (disj #{1 2 3}) ; disjoin nothing 
#{1 2 3} 

user=> (disj #{1 2 3} 2) ; disjoin 2
#{1 3} 

user=> (disj #{1 2 3} 4) ; disjoin non-existent item
#{1 2 3} 

user=> (disj #{1 2 3} 1 3) ; disjoin several items at once
#{2}