=====Boolean Operations on Page Listings=====
The possibility to combine two or more listings of pages before output in such a way as to generate a new list based on a selectable operator like
~-**+**: Intersection -- display only pages that are in both (or all) listings
~-**""*""**: Union -- display all pages belonging at least to one listing
~-**-**: Difference -- display all pages belonging to one but not the other listing

After thinking more about this: Wikka needs on one side a generic output action which accepts some content to be displayed.
%%
{{output content="some content here" col="3"}}
%%

As it wouldn't be convenient to manually enter content, Wikka needs some kind of database query facility (which must not be SQL). Currently, Wikka supports listings of pages according to:
~-category
~-last time changed
~-last time commented
~-page tag
~-linked by (and, as simple, linking to)
~-not linked by any
~-linked by pages but not existing
~-owner

Combining these elements should allow something like recent changes of a category, a list of pages that are in two or more categories, all pages linked by the homepage in alphabetical order, and so on. This means two things: Wikka must be able to retrieve different collections of pages, and Wikka must be able to sort them in different ways before the result will be displayed.

to be continued