DData revision (isSubsetOf / shorter names)

[Christian Maeder]

Daan Leijen wrote:
>>> I vote for consistently using isXXXX for predicates.
>>
>> this would (only) affect "null" (maybe "isEmpty" again) and "member".
 >
> So it is agreed: we use "isXXXX" :-)

does "isIn" meet someone's taste for "member" then? (I would not like 
"isMemberOf")

Christian