DData revision (isSubsetOf / shorter names)

[Christian Maeder]

Christian Maeder wrote:
> does "isIn" meet someone's taste for "member" then? 

or "isElem" (again I'ld find "isElementOf" too long)

It would be nice if there were consistent names for lists, sets, maps 
and bags, simply to easy remembering a name. Since the list names are 
fixed it would be okay for me to have "null" and "elem" (and "subset") 
for sets as well.

Alternatively, is it possible to add new names to the Prelude, e.g. 
"isEmpty" for lists?

Christian