The difference between 2 collections, collection2 substracted from collection1

Example: for A,B,C,D and B,C,D,E,F the difference is A

Set{'a','b','c','d'}->difference(Set{'b','c','d','e'})
= 'a'

Set{'a','b','c','d','e'}->difference(Set{'b','c','d','e'})
= 'a'

Set{'a','b','c','d','e','f'}->difference(Set{'b','c','d','e'})
='a','f'

Set{'a','b','c','d','e','f'}->difference(Set{'a','b','c','d','e','f'})
= empty

Set{'a','b','c','d','e','f'}->difference(Set{'a','b','c','d','e','f','g'})
= empty

Set{'a','b','c','d','e','f','g'}->difference(Set{'a','b','c','d','e','f'})
= 'g'