It is coherent now with the SentenceContext::is_inside(int abs_pos)
- SentenceContext::is_inside(Position pos) is added, while
Position::is_inside(SentenceContext) is removed

Previously it would mandate agreement was met on all categories
in a tag with enough categories, if such tag was found.
Now it checks for agreement only on the number of categories that
actually match, including the case of zero categories matching
(the latter does meet the weak agreement).

Clearer and better performing version, also more likely to actually work as expected :)

Note: still not implemented internals of agreement

The design doc for llook/rlook was incorrect, swapping the semantics per the updated docs and swapping argument order for llook.

The agreement stuff isn't implemented yet.

The concept has been reworked a bit.