WISH: data set function could use /case to differentiate between word types

[Oldes]

Carl was thinking about it years ago: http://www.rebol.net/r3decide/0003.html
In Red it is already implemented:

>> union [a b] ['a 'b]
== [a b]
>> union/case [a b] ['a 'b]
== [a b 'a 'b]
>> difference ['a 'b] [a b]
== []
>> difference/case ['a 'b] [a b]
== ['a 'b a b]