DData revision (isSubsetOf / shorter names)

[JP Bernardy]

> > Now, where "isEmpty" is replaced by "null", I'ld
> rather see "subset" 
> > instead of "isSubsetOf", particularly also because
> 
> > "isProperSubsetOf" is so long.
> 
> I vote for consistently using isXXXX for predicates.

Maybe I should remind the main guideline I used for
naming: follow the naming scheme of "Data.List" as
close as possible, for maximal consistency inside
hierarchical libraries.

This justifies "isSubsetOf" (modelled after
"isPrefixOf"), and "null".

I notice that "null" is not classified as a predicate
in Data.List documentation. "isEmpty" can always be
added, but then, it would be logical to add such a
function for lists too.

Cheers,
JP.

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