Set.intersection

[Christian Maeder]

Christian Maeder wrote:
> When trying to use 'Set.intersection s $ singleton e' I just noticed 
> that intersection is not left-biased (see below)!

Set.intersection is neither left- nor right-biased but biased towards 
the smaller set. I think that needs to be changed (and that may require 
a function splitLookup).

At least the documentation should be adjusted as long as the properties 
for such pseudo-sets do not correspond to those of maps.

Christian